The probe has a triple tube structure, in which an eluate from a liquid chromatograph flows through an innermost flow passage, a matrix solution flows through a flow passage outside the innermost flow passage, and the air or acetone flows through an outermost flow passage. Before analysis, acetone is flowed to rinse the matrix compound deposited in the previous analysis and clean the tip portion of the probe, and then the air is flowed to evaporate the rinsing solution.