A mass spectrometry device includes a vacuum container in which an ionizer, a mass separator and a detector are arranged, and includes an opening-closing portion that is provided at the vacuum container and switches between a locked state of being locked in a close state and an unlocked state by a difference in pressure between inside and outside of the vacuum container, and a cooling gas introducer that introduces a cooling gas for cooling the ionizer into the vacuum container such that the opening-closing portion is kept in the locked state.