An EUV light source apparatus and method for producing EUV light, which includes a plasma generation chamber for generating EUV plasma; an EUV light collector having a reflective portion irradiated by EUV light produced in the EUV plasma; a target sample having reflective portion comprised of the same materials as the EUV light collector reflective portion; a first EUV detector for detecting EUV light produced in the EUV plasma; and a second EUV detector for detecting EUV light reflected from the target sample.