Abstract: A gas laser includes a laser gas circuit having a by-pass line. The by-pass line includes a condenser for removing water vapor. During normal operation, the laser gas flows through the by-pass line as well as through the remaining portion of the laser gas circuit so that the water vapor portion in the laser gas flowing through the by-pass line is partly removed by the condenser. For the purpose of defrosting the condenser, the by-pass line is isolated from the rest of the laser gas circuit and, if required, is evacuated several times and rinsed with fresh laser gas. During normal operation, the gas laser can be operated with a substantially lower fresh gas supply than piror art circulating lasers.