PURPOSE: To simplify execution and shorten a term of works by installing at a site a precast concrete block which has been manufactured in advance in a factory or the like and integrally forming a groove, a filter tank and a water reservoir.

CONSTITUTION: A filter tank 2 communicating with an intake port 6 is placed in parallel to a groove 1 with the intake port 6 formed in parallel to a water flow direction, while a water reservoir 3 communicating with the filter tank 2 is placed in parallel to the filter tank 2. This execution is performed by installing at a site a precast concrete block for filtration and intake manufactured in advance in a factory or the like and integrally molding the groove 1, the filter tank 2 and the water reservoir 3. Thus tasks on the site such as installation of a frame, arrangement of bars, concrete placement and its curing can be eliminated.