PURPOSE: To enable easy checkup of the position of alight flux at the outlet of an integrating sphere at the time of measurement of such a body to be inspected s to change the optical axis or at the time of maintenance service and to prevent a mistake in measurement, by a construction wherein an opening- closing window for checking up the position of the light flux is fitted in the direction of advance of a light from the exit of the integrating sphere and it is made to open at the time of checkup and to close at the time of measurement.

CONSTITUTION: An opening-closing window 10 which can open and close in the direction of advance of light is fitted in the vicinity of the exit of an integrating sphere 5 and it is made to open at the time of checkup and to close at the time of measurement. A body to be inspected is set in a measuring unit and thin paper is placed at the exit of the integrated sphere. Thereby it is made possible to look directly at and check up with ease the position of the flux of the light emitted from the body to be inspected, at the exit of the integrating sphere 5, and mistake in measurement can be eliminated.