PURPOSE: To provide a biological treating device of sewage in which carriers are evenly distributed and with the denitrifying rate increased.

CONSTITUTION: Carriers on which microorganisms are immobilized are suspended in a single tank to constitute the biological treating device of sewage. The inside of the tank 1 is divided by a partition plate 2 into an anaerobic part B and an aerobic part A, the partition plate 2 has an opening C through which a suspended carrier 8 on which microorganisms are immobilized is passed at both left and right ends, a means 5 for transferring and circulating the carrier 8 is attached on at least one end of the opening C, and a sewage introducing means 4 is furnished to the anaerobic part B. Accordingly, an oxygen-contg. gas supply means 6 and a treated water discharge means 10 having a mechanism 9 to prevent the outflow of the carrier are arranged respectively to the aerobic part A, and a forced water current forming device such as a propeller agitator, a pump or a air-lift device is preferably used as the means 5.