The present invention provides a compressor having casings fixed together in a manner allowing the compressor to have a reduced sized as well as reusability.The compressor according to the present invention comprises a plurality of casings (1a, 1b) each having an open end designed to be brought into butt contact, the casings being arranged with the open ends in butt contact with each other, thereby constituting a pressure vessel enclosing a compressing unit; a flange-like projection (3) formed on the outer circumference of the open end of each of the casings, the projections of the casings butted together forming a pair; and a plurality of fixing members (4a to 4c) each in a shape corresponding to a section which would be obtained by dividing the open end of the casing, and having a groove inside, the groove being designed to engage with the paired projections butted together. The open ends of the casings butted together are fixed by the fixing members arranged to surround the casings, each in engagement with the paired projections, and welded to each other.