A rotation angle detection device has a magnetism detection unit, a magnetic flux generation unit for applying magnetic flux to the magnetism detection unit, and a magnetic flux alteration unit provided for the magnetic flux generation unit. A guide member is provided on the inner end of the rod member for supporting the rod member during movement of the rod member. A mixture is discharged from the discharge port. Therefore, for example, even if the quantity of light of the exposure wavelength component and the quantity of light of the non-exposure wavelength component individually fluctuate, it is possible to accurately ascertain fluctuations in the characteristics of the optical system resulting from irradiation heat.