PURPOSE: To provide a pretreatment apparatus in an ultra-pure water manufacturing process which solves problems arising from the existence of a small amount of organic substances by conducting surely the treatment of the organic substances, hardly receives the effects due to the change in the quality of raw water, and obviates the need for the installation of special equipment for the biological reaction.

CONSTITUTION: A biological reaction tank 21 in which a tank main body 22a equipped with an aeration means is filled with a dense fibrous fixing material and oligotrophic microorganisms are immobilized on the surface of a chenille- shaped immobilizing material, a biological filtration tank 2 in which the tank main body 22a is filled with porous glass 22c and oligotrophic microorganisms are immobilized on the surface of the porous glass 22c, a line mixer 23 for sterilization, a line mixer 24 for coagulation, and a pressure filter 25 are arranged in sequence along a passage. An ultrafiltration backwash pipe 41a, which is a backwashing means, is attached to the pressure filter 25, and backwash water of the ultrafiltration backwash pipe 41a is returned to the biological reaction tank.