1. An extreme ultraviolet light sensor unit comprising: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 and the purge gas to pass therethrough, the plasma light including the extreme ultraviolet light to be made incident on the mirror, the purge gas supplied from the purge gas supply unit, the pipe part having an opening serving as a light entry port of the plasma light, the pipe part allowing the plasma light entering from the opening to pass therethrough toward the mirror and allowing the purge gas flowing to the space between the mirror and the filter to flow out of the opening,
wherein the gas outflow port of the purge gas supply unit is provided at a position not facing the filter.