PURPOSE: To rationally treat night soil while keeping the quality of the treated water by remarkably reducing the amt. of an inorg. flocculant to be injected after biological treatment and remarkably reducing the output of flocculated sludge.

CONSTITUTION: An Mg compd. 2, excess activated sludge 3, flocculated and settled sludge 4, and a high molecular flocculant 6 are added to night soil 1, and then the night soil is introduced into a solid-liq. separator 7 wherein the night soil is separated into separated water 8 and concd. sludge 9. The separated water 8 is introduced into an activated sludge process device 12, the activated sludge is separated therein, and then the inorg. flocculant 14 is added to flocculate and settle the suspended matter. Namely, the amt. of the inorg. flocculant to be injected can be remarkably reduced in the flocculation-sedimenting process in the treatment of night soil as compared with the conventional process. Accordingly, the running cost can be reduced, the output of flocculated sludge is significantly reduced, and the treatment and disposal of sludge can be rationalized.