The present invention is a filter mechanism provided in a pump or the like, and the purpose thereof is to provide a filter mechanism that can eliminate foreign matter adhering to the surface of a filter and thereby can prevent clogging of filters because of foreign matter. According to the present invention, an area toward the inside in the direction of the radius of the filter is linked to an intake opening (235) of the pump (for example, a trochoid pump (22)) and, via the pump (22) to a pump ejection opening (trochoid pump ejection opening (215)), and an area between a filter (5) and (an inside wall surface (3i) of) a first casing (3) is linked to a low-pressure ejection opening (discharge opening (237)) for a working fluid.