PURPOSE: To prevent the clogging between contact materials by making the specific gravity of the contact materials in a purifying treatment space almost same to that of water to be treated in a wet state and setting the receiving amount of the contact materials so that the contact materials become a floating state only when the water level of water to be treated rises to a predetermined water level or higher.

CONSTITUTION: Contact materials 15 for the adhesion of bacteria are received in the purifying treatment space 20 surrounded by wall parts 13, 14 so as to leave a space above them. The specific gravity of contact materials 15 is set so as to be almost equal to that of water to be treated in a wet state or so as to be floatable in the water to be treated. The receiving amount of the contact materials is set so that the contact materials sunk in water by the wt. of the upper contact materials are held to a densely packed sedimentation state at a usual time and become a floated state only when the water level of the water to be treated rises to a predetermined water level or higher. As a result, the washing in the treatment space can be executed by the energy of water level difference such as tide level energy.