A mass spectrometer including a sample attaching member of attaching a sample, an ionizing chamber including an introductory port of the sample attaching member and an ionization source of generating a sample ion, a vacuumed chamber having a mass analyzer of analyzing the sample ion, and an opening/closing mechanism provided between the ionizing chamber and the vacuumed chamber, in which the opening/closing mechanism is controlled from a closed state to an open state after introducing the sample attaching member into the ionizing chamber to thereby enable to perform ionization with inconsiderable fragmentation at a high sensitivity with a high throughput.