A gas laser apparatus including a laser oscillating section including a medium circuit allowing a medium gas to flow therethrough under pressure, and a gas-composition adjusting section for adjusting the composition of the medium gas flowing through the medium circuit of the laser oscillating section. The gas-composition adjusting section comprising a gas supply section for supplying several types of medium gases having different compositions to the medium circuit of the laser oscillating section with a flow rate of each of the medium gases being adjustable; a gas exhaust section for exhausting the medium gas from the medium circuit of the laser oscillating section; and a control section for controlling the gas supply section and the gas exhaust section, to adjust the composition of the medium gas flowing through the medium circuit, in accordance with an oscillation condition including at least one of an attribute of an object matter acted on by a laser beam oscillated by the laser oscillating section and the accuracy of action of the laser beam.