To provide a liquid chromatograph/mass spectrometer for enhancing signal strength and improving analytical sensitivity, by adjusting new parameters that have not been used before, other than the sample amount and the applied voltage.

The liquid chromatograph/mass spectrograph atomizes a liquid sample, supplied from an LC section by a spray 15 in an ionization chamber 11 and applies a high voltage to a generated mist-like sample, and then ionizes the sample and introduces it into a mass spectrometer chamber 14. In the liquid chromatograph/mass spectrometer, a supply channel 156 between nitrogen gas 158 and oxygen gas 160 is connected to the ionization chamber 11, adjustment mechanisms 157 and 159 for adjusting the composition ratio between the nitrogen and oxygen gases in the ionization chamber 11 are provided, thus performing ionization with optimum gas composition ratio.