To provide a water treating device in which a specified amount of microorganism is deposited on a fluidized carrier in a short time so as to reduce the start-up time for operation.

In the water treating device filled with a carrier which flows in water while holding microorganism in the treating tank, the carrier comprises a carrier body made of plastic particles and one kind or a plurality of surface modification substances selected from iron hydroxide, aluminum hydroxide, phosphates, calcium salts and magnesium salts deposited on the plastic particles. Also the carrier body is made of sponge-like particles.