To provide a gas engine working machine using a cassette cylinder such as a fuel vessel for a cassette cookstove.

A gas engine working machine 1 includes a working apparatus driven by an engine regardless of the direction of a machine body, is provided for installing a gas cylinder 30 in an inverted shape, and includes a gas cylinder rotating chamber 10 for rotatably holding the gas cylinder 30 around a vertical axis (40) to the longitudinal direction of the gas cylinder 30. The gas cylinder rotating chamber 10 is provided with an installation holder 21 for installing a flange 31 of the gas cylinder 30, and a position of the gas cylinder 30 is maintained in an inverted state by the dead weight of liquefied gas 35 in the gas cylinder 30 and the installation holder 21, and an L-shaped takeout pipe 33 is orientated in a direction in which the pipe is always immersed in the liquefied gas 35.