PURPOSE: To effectively separate carrier or activated sludge by installing a freely rotating cylinder provided with a filter medium, which has openings with smaller diameter than the size of carrier, on the outer circumference of the cylinder at a bottom of an aerator and allowing a treated water in the aerator to flow out to the outside of the aerator via the above-mentioned cylinder.

CONSTITUTION: The freely rotating cylinder 20 provided with the filter medium, which has openings with smaller diameter than the size of the carrier, on the outer circumference of the cylinder is installed at the bottom of the aerator of the bioreactor 8. Then the treated water in the aerator 8 is allowed to flow out to the outside of the aerator via the cylinder 20. Thus, the carrier or the activated sludge is efficiently separated and the clogging of the filter medium of the cylinder 20 is prevented.