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