PURPOSE: To keep a flow rate of sample liquid to be introduced to an ion source constant even when viscosity of a moving phase is changed in a liquid chromatographic mass spectrometer having an pneumatic splitter.

CONSTITUTION: A moving phase adjusted in a prescribed mixing ratio by means of a gradient controller 18 is supplied to an ion source through a separating column 1 and an introducing pipe 3. A discharge pipe 5 is connected to the introducing pipe 3, and excessive sample liquid is divided by means of the discharge pipe 5 so as to be discharged. Switching cocks 22a and 22b, resistance pipes 24a and 24b between them and a pipe 23 are arranged in the discharge pipe 5, and the resistance pipes 24a and 24b or the pipe 23 can be connected selectively to the discharge pipe 5 by means of the switching cocks, and an operation as an air pressure splitter formed by using the pipe 23 and an operation as a splitter base on passage resistance formed by using the resistance pipes, can be carried out.