Tansen was a form of tax in Japan's Middle Ages.
It was adopted for the first time in 1241 by the Kamakura bakufu (Japanese feudal government headed by a shogun). This was a temporary tax imposed on specific regions (usually by provinces) to fund national events or construction of shrines and temples.
The principle was to pay in cash, and the amount was calculated in proportion to 'the scale of farming or the size of rice fields.'
Kodendansen (public tansen) was tansen imposed on the basis of the number of public fields recorded in the Otabumi (state land register).
In the Muromachi period, this tax was imposed so often that it gradually transpired into a permanent tax. A magistrate would generally be sent by the bakufu to the tax-imposed province for tax collection, but sometimes the bakufu would have a provincial governor act on its behalf. Sometimes a magistrate or a provincial governor would add a top-up above the tax they calculated and put the difference in their pockets. This encouraged the court nobles, shrines and temples, and the bureaucratic groupings in the Muromachi bakufu, such as hokoshu (a military post in Muromachi Shogunate) and bugyoshu (group of magistrates), to acquire the privilege from the bakufu either to be exempted from tansen or to pay directly to the bakufu's magistrate in Kyoto, who was in charge of monetary and rice payment to the bakufu.
At the same time, provincial governors and owners of manors began collecting shitansen, which was private tansen. Landlords kept tansen registers and used them as references for collecting tansen and records of tax collection in their estate. Shitansen developed into a form of feudal landlord-tax imposed by provincial governors and Sengoku daimyo (Japanese territorial lord in the Sengoku period).