A gas laser apparatus includes an actual laser output acquiring unit that acquires a first actual laser output at a predetermined laser output command after passage of a predetermined time from issuing of a first laser gas pressure command and acquires a second actual laser output at the predetermined laser output command after passage of the predetermined time from issuing of a second laser gas pressure command smaller than the first laser gas pressure command and a determining unit that determines whether the composition ratio of a laser gas in a gas container is normal or not by comparing the first actual laser output with a first reference output and comparing the second actual laser output with a second reference output smaller than the first reference output.