To provide a biological toilet system enabling a bowl to be repeatedly used by preventing breeding environment of biological bacteria from degenerating due to an excessive amount of water in a digestion tank.

The biological toilet system is provided with a bowl use sensing means 22 for sensing the use of the bowl 12 and a water amount controlling means 30b for regulating supply of water inside the tank 13 by an opening/closing valve 36 from a signal of the sensing means 22 according to a frequency of the use of the bowl 12. Since the supply of water inside the tank 13 is regulated according to the frequency of the use of the bowl 12, the environment adaptable to breed the bacteria inside the tank 13 can be maintained, thus enabling the bowl 12 to be repeatedly used for a long time.