A laser apparatus that can reliably prevent the formation of condensation is disclosed. In the laser apparatus, the temperature of cooling water supplied into the interior of the laser apparatus is controlled within a first predetermined temperature range during laser oscillation, while at the same time, continuously performing dehumidification so that the relation (dew point of air inside laser apparatus)+(first predetermined temperature difference)≦(cooling water temperature) is maintained. The dew point of the air inside the laser apparatus can be obtained by a computing unit from the humidity and temperature of the air.