To average the flowing amt. of sewage by varying the ratio of the outflow amt. of sewage to the inflow amt. thereof by simple constitution.

A storage tank 2 is demarcated by first and second partition walls 4, 10 to form an anaerobic tank 5 and an aerobic tank 6 and a sedimentation tank 11. A disinfection tank 13 is demarcated at the upper part of the sedimentation tank 11 by a wall part 14 formed so as to enable sewage 3 to overflow the sedimentation tank 11. A raw water pipe 7 allowing sewage 3 to flow in is connected to the anaerobic tank 5 and a discharge pipe 19 is connected to the disinfection tank 13. A small hole like communication hole 16 of the max. flow amt. almost small to the day average flow rate of the sewage flowing into the storage tank 2 is formed to the wall part 14 and, when sewage 3 flows in an amt. less than the max. flowing amt. of the communication hole, sewage passes through the communication hole from the sedimentation tank 11 while it is subjected to purifying treatment in the anaerobic and aerobic tanks 5, 6 to flow in the disinfection tank 13. When sewage 3 flows in an amt. more than the max. flowing amt. of the communication hole, the water level in the storage tank 2 to be stagnated in the sedimentation tank 11 to be delayed in its flow rate to be allowed to overflow the wall part 14. An outflow amt. is averaged.