PURPOSE: To prevent the damage and maldistribution of a porous adsorbent, to conduct decomposition treatment by iron oxide, and to improve a purification capacity by installing a lid on the upper opening of a purification chamber and a net basket in the purification chamber, and packing purification material in the net basket.

CONSTITUTION: A water flow W is accelerated by a step part F' and passes through a lid 4 which closes the first. purification chamber 1. In this operation, large garbage, etc., are removed by the lid 4, and enough air is supplied to the water flow W. Small dirt contained in water which is introduced into the first purification chamber l through the lid 4 is adsorbed with a porous adsorbent 8 comprising charcoal in a net basket 7. The water flow W, which is purified to a certain extent, is introduced into the second purification chamber 2 from a communication port 6. In this process, earth and sand contained in the water flow W precipitate on the bottom wall 11 of the first purification chamber 1. The water flow W, which is introduced into the second purification chamber 2, is treated repeatedly as in the first chamber 1 and discharged from a lid 5. In this way, the damage and maldistribution of the porous adsorbent 8 is prevented, and also decomposition treatment by iron oxide is conducted.