To prevent an adverse influence from being exercised on a system and to prevent the breakage of a valve system by a method wherein a contaminant is prevented from mixing in a hydraulic system.

A screw is fitted in the inlet part 3A of an oil pump 3 to effect the forced feed of hydraulic oil to a hydraulic cylinder device in such a state that an eye connector 15 arrange at the end part of a hydraulic piping 14 from an hydraulic pressure source is coupled to an outer peripheral surface. An eyebolt 16 in which an oil passage (a central hole 16b) communicated with the hydraulic piping 14 and an eyebolt 16 is formed is provided. A mesh filter 7 molded in a cylindrical shape is inserted in the oil passage (a central hole 16b) and incorporated therein.