PURPOSE: To control a pump head to reduce a pressure fluctuation without using a detecting device generating high-speed suction and high-speed discharge completion signals.

CONSTITUTION: When a controller 12 controlling the reciprocating motions of a piston at a pump head 2 receives a suction start signal from a detecting means 10 after the discharge stroke at a constant-piston speed, it increases the piston speed to perform suction and discharge for the period W determined by the equation W=A(f0-f)2+(α+B)P+C, where (f) is the rotating speed of the motor of the piston determined by the discharge flow quantity of the discharge stroke at the constant-piston speed, P is the discharge pressure of the discharge stroke at the constant-piston speed, and α is the compression ratio of a solvent. The discharge stroke at a constant speed is again resumed.