PURPOSE: To make clogging hard to generate by charging contact materials whose specific surface areas become large as the cross-sectional areas of respective unit water channels become large in the respective water channels and allowing the flow of the water to be treated in each of the unit water channels to go toward the part having a large cross-sectional area from the part having a small cross-sectional area.

CONSTITUTION: A partition bank 1 is constituted of the inner wall 3 facing to a semi- closed water area 2, the outer wall 5 facing to a peripheral water area 4 and the contact materials 11 for the adhesion of bacteria charged in the water channel 6 formed between the inner and outer walls. The water channel 6 between the inner and outer walls is partitioned by a middle partition 8 to provide an outside unit water channel 10 gradually becoming wide toward the end parts of the partition bank 1 and an inside unit water channel 9 gradually wide toward the central part of the partition bank 1. The contact materials 11 whose specific surface areas become large as the cross-sectional areas of the unit water channels become large are charged in the respective unit water channels and, further, the flow of the water to be treated in each of the unit water channels is set so as to go toward the part having a large cross-sectional area from the part having a small cross-sectional area. As a result, excellent purifying function is provided and clogging becomes hard to generate.