A method for cleaning an optical element of an extreme ultraviolet light source device for removing, from the optical element in a chamber, scattered matter generated together with extreme ultraviolet light by plasma formed through laser beam excitation of a target in the chamber, the method which comprises: making the scattered matter generated by the plasma no larger than nanosize by using solid tin as the target and using a CO2 laser as an excitation source of the solid tin; and imparting, to the scattered matter no larger than the nanosize adhered to the optical element, an effect of overcoming the adherence of the scattered matter.