A laser device includes an excitation light source, a resonator which receives excitation light from the excitation light source and generates laser light, an absorption cell to which the laser light is emitted, a light converter which converts the laser light passing through the absorption cell to a light output signal, a third order differential lock-in amplifier which generates a third order differential signal of the light output signal, and a controller. When a predetermined waveform of the third order differential signal is detected, the controller includes a return controller that determines a return direction of a resonator length based on the predetermined waveform and a resonator length controller that changes the resonator length to the return direction.