A mass spectrometer provided with an ionization chamber (10) in which ionization is performed on a sample by laser ionization, includes an opening part (12) that is provided on a side wall of the ionization chamber (10), and includes a door (13); a ventilation port (14) provided in a wall of the ionization chamber (10), which is opposite to the opening port (12); and a gas supplier (64), (67) for supplying high-pressure cleaning gas to the ionization chamber (10) through the ventilation port (14). In this configuration, the high-pressure cleaning gas flows into the ionization chamber (10) from the gas supplier (64), (67) while the door (13) is opened, thereby blowing up particles including fragments of bacterial cells, which are piled up on a floor of the ionization chamber (10), and/or sweeping particles floating near the floor, so as to discharge the particles to the outside.