Slider-crank mechanisms are used as a rotational-to-linear motion transformer in positive displacement pumps, because such mechanisms are effective for generating high discharge pressure. The stroke of slider-crank mechanism in a water pump is twice the crank length and the connecting rod must be sufficiently long to avoid problems caused by the side force, thereby, limiting the extent to which a positive displacement pump can be miniaturized. Small pumps with high discharge pressure are expected to be used as the power source of a hydraulic actuator. In this study, a mechanism wherein eccentric cams replace a crank and a connecting rod is proposed, and the operating principle and characteristics of this mechanism are investigated. The mechanism has a stroke that is four times the crank length, and it is designed with reference to an Oldham coupling mechanism. Replacing a crank with an eccentric cam is expected to reduce vibration by reducing fluctuations in the position of the center of gravity, but a concern is that using an eccentric cam will cause friction loss. An experimental device is manufactured to investigate the driving torque and the vibration characteristics.