An EUV light source device properly compensates the wave front of laser beam which is changed by heat. A wave front compensator and a sensor are provided in an amplification system which amplifies laser beam. The sensor detects and outputs changes in the angle (direction) of laser beam and the curvature of the wave front thereof. A wave front compensation controller outputs a signal to the wave front compensator based on the measurement results from the sensor. The wave front compensator corrects the wave front of the laser beam to a predetermined wave front according to an instruction from the wave front compensation controller.