An operation mechanism for a variable displacement hydraulic pump. The hydraulic pump is disposed in a housing filled with fluid. A capacity regulating member is provided on the hydraulic pump in the housing for changing the discharge amount of the hydraulic pump. An operation member is operationally connected to the capacity regulating member. The operation member is to be manipulated outside the housing. A resistive device is interposed between the operation member and the capacity regulating member for making the motion of the capacity regulating member slow. The resistive device is an assembly unit comprising a casing, a piston slidably disposed in the casing, and fluid filled in the casing. The casing is fitted into the hole provided in the housing.