To enable to detect in real time when the irradiation distribution characteristics have worsened at or beyond the middle focal point in an extreme ultraviolet light source device, and enable to correct the worsened irradiation distribution.

In an EUV light source device, a cylindrical detection means 20 is installed for detecting EUV beams that do not focus on a middle focal point f. The EUV beams that focus on the middle focal point f pass through an opening 20a of the detection means 20, so that EUV beams that do not focus on the middle focal point f can be detected even during exposure processing. The irradiation distribution of the EUV beams that do not focus on the middle focal point f correlate with the irradiation distribution of the EUV beams that focus on the focal point f and enter the exposure machine, hence, from the worsening of the irradiation distribution of the EUV beams obtained through the detection means 20, it is possible to know the worsening of the irradiation distribution of the EUV beams that focus on the focal point f. In addition, it is possible to improve the irradiation distribution of the EUV beams that enter the exposure machine by moving a light-focusing mirror 6 so as to improve the irradiation distribution of the EUV beams.