To flexibly cope with an attachment or an addition of a piston pump to a hydraulic circuit not attached with a suction strainer from the beginning, to reduce costs and the number of components by eliminating the need to provide a suction filter in an inlet passage, and to efficiently maintain the suction strainer without a large scale work.

Nearing the suction port 3 of a hydraulic pump 1, the suction strainer 10 removing refuse in pressure oil sucked in from a suction port 3 of the hydraulic pump 1 is attached to the hydraulic pump 1.