A laser device comprises: first and second laser excitation regions that are disposed in series each other; a first power supply unit (PSU1) that injects a first energy into the first laser excitation region; and a second power supply unit (PSU2) that injects a second energy into the second laser excitation region, wherein the first power supply unit injects a predetermined excitation energy equal to or higher than a critical injection energy at which laser oscillation starts into the first laser excitation region as the first energy and the second power supply unit injects energy between a preliminary excitation energy needed for preliminary discharge and the critical injection energy into the second laser excitation region as the second energy, so as to control laser power. Consequently, the laser device can stably output very low laser even though it is a high power laser.