PURPOSE: To provide a packed tank with filtering medium wherein a filtering medium can be easily taken in and out of the tank.

CONSTITUTION: A filtering medium 3 is filled in a net-like body 2 such as a bag-like or a box-like shape with a freely flowable space and this net-like body 2 is immersed in a tank 1. In the second invention, the part packed with filtering medium like this is provided on the downstream side of the precipitation part and a floating substance with a relatively large particle size is removed at the precipitation part and a fine floating substance is removed by the use of the filtering medium in the net-like body 2.