To effectively defoam scum without using clean water and increasing an amt. of treated water by using treatment process water as crushing water of scum generated in a sewage treatment process.

An underwater pump 4 is provided in a pump tank 2 arranged to a part of a treatment tank 1 through a flooding weir 3 to suck water in the treatment tank 1 and the scum flooding along with sewage in the treatment tank 1 is crushed by the impeller of the underwater pump 4 and the crushed scum is circulated to a nozzle 7 arranged so as to inject water to the scum floating on the surface of water in the treatment tank 1 and the scum floating on the surface of the water is crushed by the injection of water in the treatment tank 1 to be transferred to the pump tank 2.