PURPOSE: To send a trace of a constant flow rate accurately with good reproducibility, by carrying out a piston drive by liquid pressure.

CONSTITUTION: The area of the rear end face which is in contact with the moving phases A1, B2 of pistons 9, 9' of liquid sending pumps A3, B4, is made smaller than that of the rear end which is in contact with a liquid 30 for driving. Each pump A3, B4 is filled with each phase A1, B2 and suction valves 22, 22' are closed and then, jetting valves 22, 23 are opened. Further, valves 16, 16', 15, 15' are closed and valves 17, 17' are opened. Next, liquid sending of a pump 28 for driving is started and at the same time, a valve 14 or 14' is opened. The phases A1, B2 are sent out at an optional proportion by opening and closing the valves 14, 14' alternately for every prescribed time. Both phases are mixed in a mixer 5 and the mixture is introduced into a column 7 through a sample injector 6 and then, is discharged through a discharge device 8.