A liquid chromatography/mass spectrometry apparatus combines a chromatograph having a liquid delivery part for delivering mixed multiple solvents, and a mass spectrometer having an ionization part for spraying and ionizing. The apparatus has a storing part for storing a relationship between a mixture ratio of the multiple solvents in the liquid delivery part and an optimal or nearly optimal voltage applied to the ionization part, a ratio information acquiring part for computing the mixture ratio over time, a voltage value computing part for computing the optimal or nearly optimal applied voltage at any point, and a voltage controlling part for changing the voltage applied to the ionization part over time. By virtue of the ability to adjust the applied voltage during analysis, the ionization efficiency is always high, and the analysis can be performed with high sensitivity for all of the multiple components in the liquid sample.