To enhance concentration properties by surely separating water from sewage in the body of an air mixing pipe in which sewage is made to be an aerobic state by surely mixing air with sewage.

In separators 50 for sewage treatment, sewage passages 71 are formed in the insides of sewage separation pipes 69 which communicate sewage inflow port parts 67 and sewage outflow port parts 68. The outflow port parts 72 of separated water communicate with the sewage passages 71 through separation bodies provided in the sewage separation pipes 69. The sewage outflow port parts 58 of the body 49 of an air mixing pipe 47 for concentration are connected to the sewage inflow port parts 67 of the separators 50 for sewage treatment. The body 49 of the air mixing pipe is equipped with a sewage supply pipe and an air supply pipe and a sewage flow path is disposed between the outer periphery of the air supply pipe and the inner periphery of the sewage supply pipe. Also, a negative pressure generation space is formed in the outside of an air outflow port part in the air supply pipe on the downstream side in the direction of sewage flow in the sewage flow path. A mixing flow path is disposed in the sewage supply pipe among the negative pressure generation space, the sewage flow path around it and the sewage outflow port parts 58.