PURPOSE: To shorten the lengthwise dimension of the pump by a constitution wherein a plunger is inserted into a cylinder attached to a crank case and a water path, communicating a suction port with a delivery port, is provided at the tip end of the plunger.

CONSTITUTION: When the plunger is reciprocated by a proper driving source through a crank 6 and a connecting rod 7, the plunger 4 is moved in the advancing stroke thereof under a condition that a suction valve 13 is closing the water path 12, therefore, liquid, supplied to the tip end side of the plunger 4, is discharged against a spring 20 through the delivery port 11 under opening a delivery valve 21 by the pressure thereof. On the other hand, the tip end side of the plunger becomes vacuum in the returning stroke of the plunger and the suction valve 13 opens the water path 12. Then, the liquid of a constant volume is supplied to the tip end side of the plunger 4 through the water path 12 when the water path 12 is communicated with the suction port 10 and is discharged upon the next advancing stroke of the plunger.