Generally, in the pump driving mechanism, a pure reciprocating motion must be given to a piston to prevent water leakage and a contrivance to achieve that motion, such as a crank connection is required.
Therefore, in such conventional pump driving mechanisms as the one which has been adopted in a water feeding device 70 of, for example, FIG. 15, a driving force of an output shaft 72 of an electric motor 71 for a pump is transmitted to a driving gear 74 through a gear 73. An eccentric rotary motion of an eccentric pin 75 provided to project on the driving gear 74 is converted to a reciprocating motion through a slider 76 and piston joint 77, and a piston 78 is correctly driven to reciprocate within a cylinder 79. In such mechanism, however, a large noise has been generated when it is driven and the energy loss has been large for the following reasons. That is, there are seven places in which the respective elements slide in contact with each other, namely, between the eccentric pin 75 and the slider 76, between the slider 76 and the piston joint 77, between the piston joint 77 and the driving gear 74, between the slider 76 and the driving gear 74, between a piston rod 80 and a shaft bearing 81, between the gear 73 and the driving gear 74 and between the driving gear 74 and a reduction gear shaft 82 and the noise has been large.