Tomioka Silk Mill (富岡製糸場, Tomioka Seishijō) is Japan's oldest modern model silk reeling factory, established in 1872 by the government to introduce modern machine silk reeling from France and spread its technology in Japan. The factory is designated by the government as a National Historic Site and all its buildings are preserved in very good condition. It is located in the city of Tomioka, Gunma Prefecture, Japan, about 100 km northwest of Tokyo. It is also featured as the 'ni' card in Jomo Karuta playing cards.

Following the Meiji restoration, the new Meiji government needed export products to raise the monies necessary for the industrialization and westernization of Japan. Goods which were already produced in Japan and which could be exported for good profit included tea, silk thread and silkworm cocoons. Especially with the spread of the silkworm disease called pébrine in France and Italy, and the turmoil in China caused by the Taiping Rebellion, Japanese silk was in high demand. In 1862, shortly before the Meiji restoration, raw silk and silkworm cocoons accounted for 86% for Japan's exports. However, soon after the restoration, overproduction, and the recovery of silk producing areas in Europe and China led to a drastic fall in raw silk prices. The Meiji government countered by changing its focus to silk products, such as silk thread and cloth, to increase the value of its silk exports. Foreign merchants had previously approached the government with offers to provide funding for a silk mill but this was rejected as the government considered silk production to be a critical strategic industry; however, Japan lacked the technology to construct a mill on its own. In 1870, a government-owned model factory, the Maebashi Silk Mill, was constructed with equipment imported from Italy and with a Swiss engineer to assist. It was only on a small scale and employed 12 people. Later the same year, leaders of the Meiji government, including Ōkuma Shigenobu, Itō Hirobumi and Shibusawa Eiichi approached the French embassy and were introduced to Paul Brunat, an engineer who was working as a raw silk inspector in Yokohama to oversee a project to construct a much larger facility.

Brunat in turn hired Edmond Auguste Bastan to create a blueprint for the design of the silk mill, which was completed at the end of December 1870. Bastan was able to complete the design such a short period as he had previously designed the Yokosuka ironworks and was familiar with made of timber-framed brick buildings. Brunat returned to France on to purchase equipment and hire engineers. In the interim, Odaka Atsutada was placed in charge of the Japanese side and started procuring materials, and in March 1871, construction started. Brick was not yet in common use, so a brick kiln had to be constructed in what is now Kanra, Gunma where clay of the appropriate quality had been found.

The Tomioka Silk Mill started operations on November 4, 1872. However, due to the shortage of female workers, about 210 female workers initially operated with half of the total reeling machine. The Westerners on site was seen drinking red wine; after that, unfounded rumors spread that they lived off of blood drained from the Japanese workers, which scared off many potential workers. Odaka was then forced to hire his own daughter to help dispel these rumors. As of January of the following year, there were 404 workers, mainly daughters of former samurai. The number of workers in April 1873 had increased to 556. The life of the workers has been recorded in the diary of one, Wada Ei.

The Tomioka Silk Mill is a huge timber-framed brick building, containing 300 reels. The largest plants in France and Italy had up to 150 reels, so the Tomioka Mill was one of the largest in the world at the time of its completion. The reels also incorporated a rewinding process called re-rolling, where the raw silk was re-wound from a small frame into a larger frame. This was done as Brunat wanted to mechanically imitate the processes which Japanese craftsmen had formerly done by hand. In the case of the humid Japanese climate, once the raw silk was wound, sericin could cause the silk threads to stick together. The rewinding process, which was not necessary in the drier European climates, prevented this. Another point which Brunat took into account when special-ordering the reeling equipment in Europe was the smaller stature of the Japanese women who operated the machines.

The working environment at the Tomioka Silk Mill was progressive for its time. Brunat introduced eight-hour working days, Sunday holidays and ten-day holidays mid-year and at year-end. The workers were provided with uniforms, and as the prefectural governor, Katori Motohiko was enthusiastic on education, the women workers had access to an elementary school. However, worker turnover was high, as there was considerable social pressure against women working in a factory, and friction between women from different social classes forced to work side by side. Many workers left within three years of employment, and the need to keep training new worked added to the mill's expenses. Workers were graded according to their skill level, with a system of eight ranks introduced in 1873. Silk produced at the mill received a second place award at the 1873 Vienna World's Fair, and "Tomioka silk" became a brand name. Workers from Tomioka were also sent, or otherwise found employment at other privately owned silk mills which were subsequently built in Japan.

Brunat and other foreign engineers employed at Tomioka left at the end of 1875 when their contracts expired, and thereafter the mill was managed only by Japanese. Odaka came into conflict with his government overseers and was forced to retire in November 1876 as well, but by this time the mill was showing good profits, and in the following year, direct exports to France by Mitsui & Co. began. Plans to privatize the mill was delayed when cabinet minister Matsukata Masayoshi visited the Exposition Universelle (1878) in Paris as was told that the quality of silk from Tomioka had deteriorated considerably. On his return, Matsukata fired Odaka's successor, Yamada Noriyuki, who had been hindering reforms, and attempt to install a former official from the Home Ministry, Hayami Kenzo, as his successor. Hayami had become president of a company exporting silk, and was a strong proponent of privatization. He agreed to rent the Tomioka Silk Mill for a five-year period, but this was strenuously opposed by the Gunma Prefectural government. By 1884, when most government-owned enterprises had been sold off, Tomioka Silk Mills remained in government hands. Due to its huge size, it was beyond the means of private investors at the time. Under its fourth director, Okano Asaji, a drop in silk prices worldwide plunged the operation deeply into the red. Hayami returned as director again in 1885 and turned the situation around by opening new markets in New York and by raising the reputation of Tomioka silk by increasing quality control.

In September 1893, Mitsui & Co. raised a 121,460 Yen bid and purchased the Tomioka Silk Mill from the government. Under management of Mitsui, a second women one-story factory was established, and new, more modern, reeling machines were instructed, All of the raw silk produced at the Tomioka mill was exported to the United States. A dormitory was also constructed for workers, although about half of the workers continued to commute. Working hours were also increased to twelve hours in summer and nine hours in winter. The elementary school was retained, but after twelve hour shifts, most workers were too tired to want to attend. Mitsui maintained three silk mills in addition to Tomioka, but the combined profits was not very large, and in 1902 Mitsui sold the plants to the silk merchant and entrepreneur Hara Tomitarō for 135,000 Yen.