A laser apparatus according to one aspect of the present disclosure includes a master oscillator configured to output laser light, a plurality of amplifiers each configured to include carbon dioxide as a laser medium and amplify the laser light, a first optical path pipe configured to cover a laser optical path between the amplifiers, a gas supply port configured to supply, into the first optical path pipe, gas having lower carbon dioxide concentration than that of the air, a first carbon dioxide densitometer configured to measure carbon dioxide concentration in the first optical path pipe, and an alarm device configured to issue an alarm when the carbon dioxide concentration measured by the first carbon dioxide densitometer exceeds a preset prescribed value.