To provide a degradation determining apparatus for determining the degree of a degradation in an impurity removing member such as a chemical filter before an optical component is exposed to the impurity.

The degradation determining apparatus comprises: a light guiding section (a half mirror 11) for guiding a light from a light source (a short wavelength light source 28); a degradation determining member (a target plate 12) exposed to the impurity other than a desired atmosphere while it is irradiated with the light guided from the light guiding section; and a monitoring section (a photoelectric sensor 14 and a monitor 16) for monitoring the degree of the degradation in the degradation determining member.