To increase a microorganism holding amount of the carrier added denitrifying device and also to aerate and agitate by using a gas generated in a closed space.

This device is provided with a reaction tank 10 in which a microorganism carrier 12 is packed and also to which a raw water supply means 11 and a water discharge means 14 of a treated water are connected, a closed vessel 18 forming the closed space at an upper side of the reaction tank 10, an air diffusing pipe 21 laid on a bottom part of the reaction tank 10 and an aeration means 22 circulating and supplying the gas accumulated at the closed space at the upper part of the reaction tank 10 and agitating the reaction tank 10.