an ionization chamber 100 is provided between a liquid chromatograph unit 60 and a mass spectrometer 50 , and is formed of : an atomization means 15 ; and an ion introducing pipe 19 of which the entrance portion is created within the ionization chamber 100 in the horizontal direction that is perpendicular to the z direction and of which the exit portion is created within the mass spectrometer unit 50 . a liquid sample that has been fed from the liquid chromatograph unit 60 is sprayed in the z direction by the atomization means 15 while being ionized within the ionization chamber 100 , wherein the entrance portion has an opening in such a form that corresponds to the spread in the xy plane of the liquid sample sprayed in the z direction . the sprayed liquid sample is then fed into the mass spectrometer unit 50 while being desolvated , which effectively contributes to analysis .