Cylinder mounting mechanism is provided in a working machine for mounting thereto a cylinder case having a cassette gas cylinder accommodated therein. Mouthpiece support section is provided in the mounting mechanism for supporting a mouthpiece portion of the cassette gas cylinder in the cylinder case. Fitting recessed portion is provided in one end portion of the cylinder case and capable of receiving the mouthpiece support section, the fitting recessed portion being defined by an inner peripheral wall part of the one end portion that has a tapering wall thickness such that the surface of the wall part slants gradually outwardly toward the distal end of the fitting recessed portion.