PURPOSE: To obtain a method and an apparatus enabling execution of gradient measurement by a simple construction.

CONSTITUTION: Moving phases A and B held in vessels 11A and 11B are sent selectively to a pump 13 through a selector valve 12. The moving phase pressurized by the pump 13 is sent to a column 4 through a selector valve 14, a reservoir 15 provided with an agitator, an injector 3 and a selector valve 16. A liquid flowing out of the column 4 is supplied to an ion source 5, while a surplus part is discharged outside through a hydraulic splitter 17. Gradient measurement can be executed by filling up the moving phase A beforehand in the reservoir 15 by using the selector valve and by supplying thereafter the moving phase B to the column through the reservoir 15.