A mirror is provided with a light source window and an illumination window each establishing communicative connection between an inner face side and an outer side of a hemispherical unit. The light source window is an opening to which a light source OBJ to be measured is attached mainly. The illumination window is an opening for guiding a flux of light from a correcting light source used for measurement of self-absorption toward the inner face of the hemispherical unit. A self-absorption correcting coefficient of the light source OBJ is calculated based on an illuminance by a correcting flux of light in a case where the light source to be measured OBJ in a non-light emitting state is attached to the light source window and an illuminance by a correcting flux of light in a case where a calibration mirror is attached to the light source window.