An extreme ultraviolet light sensor unit according to one aspect of the present disclosure includes a mirror configured to reflect extreme ultraviolet light, a filter configured to transmit the extreme ultraviolet light reflected by the mirror, an optical sensor configured to detect the extreme ultraviolet light having passed through the filter, a purge gas supply unit disposed to supply purge gas to a space between the mirror and the filter, and a pipe part configured to allow plasma light including the extreme ultraviolet light entering from an opening to pass therethrough toward the mirror and allow the purge gas flowing to the space between the mirror and the filter to flow out of the opening.