PURPOSE: To discharge the constant volume of the liquid accurately and continuously without accompanying pulsation or the like by a method wherein a piston rod is connected to the shaft of a ball screw screwed to a ball nut while the ball nut is connected to a servomotor so as to be capable of being driven thereby.

CONSTITUTION: Simultaneously with putting ON of a starting button 70, the servomotor 11 begins to rotate with a set constant speed rotation, the shaft 4 of the ball screw and the piston 3 are moved to the left with a constant speed along a straight line through the ball nut 8, the piston 2 moves to the direction shown by an arrow sign S with a constant speed along a straight line, the liquid, being filled into a piston fore chamber A, is sent forcibly into a discharging valve 55 through a pipe 19, ports 25, 22 and the pipes 52, 54 as shown by double actual lines, thereafter, is discharged by a constant flow amount. Thus, the liquid may be discharged continuously by the constant flow amount without pulsation or the like by giving any set revolving number to the servomotor.