To improve the comfortability for a usage feeling and the cleanness of a toilet bowl by enabling inexpensive production and easy installation by providing a water drainage part in a water tank.

Concerning the buttock washer having a water tank 1 provided with a water drainage part 2, the fence-shaped water drainage part 2 is provided for removing excess water contained in toilet paper. Since water in the water tank 1 is gradually decreased by immersing the toilet paper in it or by natural evaporation, it is necessary to replenish water into the water tank 1. Then, water is previously put into the water tank 1 of the buttock washer and installed in the toilet. After evacuation, the suitable quantity of toilet paper is immersed in the water in the water tank 1 and the excess water is removed by the water drainage part 2 later. Then, the anus is washed by that toilet paper containing water. After the anus is sufficiently washed, water stuck on the anus is swept away by suitable quantity of toilet paper containing no water, and processing is finished.