To prevent deposition of gravel by agitation blades of a central rotary shaft using a plurality of rotary shafts.

By supplying sludge to a reactor 11 from a sludge supply port 25 and charging a chemical from above, the sludge coagulates to form floc by being agitated with the agitation blades 13a, 14a, 15a and overflows a partition plates 22. The coagulated floc is charged into a screw press from a discharge pipe 24 via a discharge pit 23. The agitation blades 13a of the central rotary shaft 13 agitate sludge liable to stay on the bottom of the reactor 11 to increase coagulation efficiency. Gravel including stones not coagulated stays at the central bottom part 16 of the reactor 11 and is pushed out toward an opposing side of the discharge pit 23 by the agitation blades 13a of the central rotary shaft 13. The gravel pushed out to the end of the reactor 11 is intermittently scraped up by the bucket 30 of a gravel discharge mechanism 26 and discharged to the outside of the reactor 11.