To solve the problems that a technique of suppressing heavy metal outflow using porous concrete is a mechanism of providing a pre-treatment tank and suppressing the heavy metal outflow in a porous concrete body after that and is an extremely complicated manhole structure, and it is needed to prevent dust and dead plants or the like from covering a surface of the porous concrete in order to efficiently adsorb heavy metals.

A porous concrete layer is installed for 5 cm to 15 cm at a lower part of a body, and a step of 10 cm to 20 cm is provided between porous concrete and a sewer pipe. When water is received, strong tractive force is generated, and a porous concrete surface is washed. As a material of the porous concrete, the one containing gravel and 5-10% zeolite is used.