A laser device 100 includes a laser oscillator 10, a dehumidifier 11, and a controller 9 controlling an operation of the dehumidifier 11. The controller 9 controls the dehumidifier 11 such that the dew point W inside the laser oscillator 10 is lower than the first dew point W1H when a monolayer or less of water molecules is adsorbed, or such that the dew point W is equal to or higher than the second dew point W2L when more than a monolayer of water molecules is adsorbed inside the laser oscillator 10, and is lower than the third dew point W2H at which the dew condensation starts to occur inside the laser oscillator 10.