Problem: To provide a mass spectrometer wherein the door can be easily opened even when the door becomes stuck to a third wall surface.Solution: A mass spectrometer comprising a door 50 can be opened and closed and a lever 60 having a straight-shaped section 60a that is rotatable about a rotation shaft that is perpendicular to a first edge, wherein the rotation in a forward direction of one end of the straight-shaped section 60a of the lever 60 by a person taking measurements when the door 50 is closed causes a male part 61 formed on the lever 60 to interfere with and become locked with a female part 62 that is formed on an enclosure 110, thus making the door 50 unopenable and unclosable while the rotation in a reverse direction of the one end of the straight-shaped section 60a of the lever by a person taking measurements creates an unlocked state that allows the door 50 to be opened and closed and the other end of said straight-shaped section 60a of the lever 60 presses against a part of a third wall surface 110c that creates a force to open the door 50.