Kasuisai Temple, under the Soto School of Buddhism, was built in 1401. Since ancient times, this temple has been a famous location among monks to practice zen meditation. Originally, this temple was named Toyoken Temple, but it is said the name was changed to Kasuisan Temple due to a story about Tokugawa Ieyasu.

The Tozen Priest of Buddhism, also the 11th Chief Priest of the Kasuisai Temple, saved Tokugawa Ieyasu when he was young after his family was assassinated. Later in life, Tokugawa Ieyasu became very powerful and gained the position of lord of Hamamatsu Castle. He invited the Tozen Priest to his castle and held a party for the priest in older to express his gratitude for aiding him when he was young. During the party, Tokugawa Ieyasu noticed that the priest had fallen asleep. In this period of time, this behavior was considered incredibly rude and it would not have been surprising if Tokugawa had ordered the priest to be killed. However, Tokugawa Ieyasu thought to himself, “The reason why the Tozen Priest is able to sleep in front of me is because he thinks of me as a real son. I’m glad he is able to sleep so deeply in my presence.”

Because of this story, the priest was then called the Kasui Priest. “Ka” in Japanese kanji means “possible” and “sui” means “sleeping.” In this sense, the name “kasui” meant that the priest was a person who could fall asleep even if it is in front of the feudal lord.

From that point on, the Toyoken Temple was renamed as the Kasuisai Temple. Even today, the story of this temple’s history is told across the nation due to the important relation to Tokugawa Ieyasu.