Before repeating the processing that an LC/MS analysis is performed after diluting a large number of pretreated media samples (S16 to S19), an LC-MS stabilization processing (S11) of supplying a mobile phase to the LC-MS and performing an analysis without a sample according to the same gradient profile as that in a culture medium sample analysis and a standard sample analysis (S13) of supplying a standard sample to the LC-MS and obtaining data for generating a calibration curve are performed. Before the LC-MS stabilization processing and before the standard sample analysis, respectively, a standby time corresponding to a required time of a sample dilution is provided (Steps S10 and S12). With this, the time of the cycle including the LC-MS stabilization processing and the time of the cycle including the standard sample analysis are made to be the same as the time of the cycle of a dilution and an analysis to a culture medium sample. As a result, the stoppage time of the mobile phase before the mobile phase is fed according to the gradient profile becomes the same in every cycle, and the staying time of the mobile phase in the degassing device becomes uniform, so that it is possible to avoid changes in the compositions of the mobile phases in the column and to achieve higher quantitativity.