To accurately detect intensity distribution of a light beam irradiated from a light beam irradiation device.

While moving a measurement instrument 50 that shields a part of a light beam irradiated from a light beam irradiation device 20 in an irradiation region 26a of the light beam irradiated from the light beam irradiation device 20, the light beam irradiated from the light beam irradiation device 20 and a part of which is shielded by the measurement instrument 50 is received, and intensity of the whole received light beam is detected. From a change in the intensity of the whole received light beam in accordance with the movement of the measurement instrument 50, intensity of the light beam shielded by the measurement instrument 50 and its position are detected, and intensity distribution of the light beam is detected. Based on a detection result, variation of the intensity distribution of the light beam is corrected.