PURPOSE: To provide a water pump which is small-sized and low in noise, and can obtain sufficient delivery pressure.

CONSTITUTION: A water pump is composed of a water supply-discharge block 3 to which a water supply path 31 and a water discharge path 32 are opened, a cylinder 4 which has a protruding part 40 loosely fitted in the water supply- discharge block 3 and is rotatingly driven round a shaft, a piston reciprocating in the cylinder 4, and a driving mechanism functioning under interlocking motion between the reciprocation of the piston 5 and the rotation of the cylinder. The protruding part 40 in the cylinder 4 is formed to be concentric with the cylinder and smaller in bore than the cylinder, and has an opening part 41 which is made to alternately communicate with the water supply path 31 and the water discharge path 32 in the water supply-discharge block 3 by the rotation of the cylinder.