To moderate pressure fluctuation and concentration fluctuation generated by switching flow passages, in an LCMS system using a flow passage switching type splitter.

A bypass flow passage 26 is attached in an upstream and a downstream of the flow passage switching type splitter 2 by tree-way joints 30, 32. When a groove 2a of the flow passage switching type splitter 2 is located in a separation flow passage 4 side, the pressure fluctuation is suppressed, since a mobile phase from a make-up pump 20 in a detection flow passage 8 flows through the bypass flow passage 26. A micro-volume mixer 28 is provided between a convergence point of the detection circuit 8 with the bypass flow passage 26 and a PDA 22. The concentration fluctuation is restrained from being generated by introducing an eluate intermittently in the flow passage switching type splitter 2, by feeding the liquid flowing in the detection flow passage 8 to the PDA 22 and an MS 24, after mixed in an upstream of a detector.