The Japanese bath descends from Buddhism, and bathing was thought as one of the most important rites by Buddhist monks. For instance, in Nara, one of the ancient capitals of Japan, large baths (big yuya) still remain at Todaiji Temple (built in 745) and Hokkeji Temple (built in 741). The bath was used effectively as an important means to protect against the cold because people had little heating in the Kamakura period (1192-1333). The public bath was formed in the Heian period (794-1192), but only the poor kuge, or aristocrats of Japan’s imperial court, used it because the rich had their private bath in their house. （Photo taken by Yuki Choso in Kyoto Studio Park.）
In 1591, shortly before the Edo period (1603-1867), a man who was called Yoichi Iseno opened the public bath at Zenigamebashi in the city of Edo, today’s Tokyo. It is said that this is the origin of the sento. Yoichi’s bath was a steam bath. (See the section below, Todana-buro.) In those days, Edo was still a small village in Kanto area. However, in 1590 Ieyasu Tokugawa settled in the land in order to build a city there. Ieyasu became the first shogun of the Tokugawa shogunate in 1603. As a result, the number of people coming to Edo city increased and the yuyas were opened one after another.
Between the early and late Edo period, most sentos were steam bath. The steam bath was called “todana-buro.” This was a kind of sauna and people took a bath by soaking only the lower half of the body and men and women took the bath together. At that time common people usually used sento because they did not have their own bath.
At the beginning, two sliding doors were fixed in the doorway to keep the hot steam inside. But it was not enough, so zakuroguchi was made to improve it. Zakuroguchi was a board that almost covered the front of bathtub from the ceiling. People had to stoop because zakuroguchi had a low entrance. One interesting theory of the origin of this strange name “zakuroguchi“ (“pomegranate gate“) is that the fruit juice of zakuro (pomegranate) was needed to clean a mirror at that time. This is in fact a play on words. Mirror is “kagami“ and to need something is “iru“ in Japanese. To stoop is “kagamu“ and to enter is also “iru“ in Japanese. So they had to ’stoop (“kagamu“) to enter (“iru“)’ a zakuroguchi just as ’a mirror (“kagami“) needed (“iru“) a pomegranate, or zakuro.’
In zakuroguchi, it was dark. When people in Edo city entered the bath, instead of just saying “Excuse me,” they often called out something such as: “Branches (= limbs) will touch you” or “I'm from the country” and then people in the bath cleared their throat to tell their presence. (See the illustrations of zakuroguchi: the above one from Sketches of Japanese Manners and Customs, by J. M. W. Silver published in London in 1867 and the below one from Great Encyclopedia Vol.10 published in Tokyo in 1932.)
Sue-buro that allowed people to soak shoulder-deep in a bath first appeared in the Edo period or the early 17th century. It was different from mushi-buro or a steam bath. Teppo-buro is a more modern bath system, which used a big vertical metal cylinder to heat the water in the bathtub. Goemon-buro was a large bathtub with an iron bottom heated by the fire below and the name was derived from Goemon Ishikawa, a bandit who was sentenced to execution by being boiled alive in a cauldron in Kyoto. People took goemon-buro by stepping on a wooden board or wearing wooden sandals not to get burnt.
The number of sento increased in Edo city as a place for socialization. The bathhouses hired yuna, female prostitutes, who took care of customers at yukaku, licensed pleasure quarter such as Yoshiwara (north of present Taito-ku, Tokyo Prefecture). Sento, then called yuya, became one of the most flourishing businesses consequently, and most owners built the second floor and started a club for men there. The Edo government banished yuna from sento, however, and forced them to work in yukaku at Yoshiwara.
Moreover, in the Meiji period, there were sentos called saisei-onsen, or “remodeled spa.” This spa used the hot water which was carried from the spa city. At the same time, a new type of sento appeared. It was calledkairyo-buro, or “reformed bath,” and the floor of the bathtub was made lower than the floor of the washing place like baths in today’s sento. In this period the abolition of zakuroguchi which had been used for a long time was promoted. It was all abolished by the end of the Meiji period. The length and width of a bathtub was about 3 meters, the depth of a bathtub was about 1.2 meters, and the washing room space was from 3.6 to 5.4 meters in length. The bathroom was separated between men and women. This style is the model of the present sento. (For more information about the history of sento, please visit the websites； all in Japanese: http://www.1010.or.jp/top.shtml