A mass spectrometer that allows easy replacement of an MCP (microchannel plate) and is enabled to secure orthogonality between an incident surface of the MCP and an ion track at high accuracy is provided. A flight tube 2 where ions fly is arranged in a vacuum vessel composed of a vacuum flange 6 and a body 1, and an MCP group 4 is attached to a tail end of the flight tube 2 via an MCP-IN electrode 3. A vacuum flange 6 is attachably and detachably attached to the body 1, and the MCP group 4, by a spring 710 provided on a circuit board 7 for detection attached to the vacuum flange 6, is urged toward an end portion of the flight tube 2 so that its orthogonality with respect to an ion flight track is secured.