To provide a double beam integrating sphere where a sample and the tangential direction of a reference white surface face the center of an integrating sphere and at the same time, a light trap is commonly used for the sample and the reference white surface.

The integrating sphere 10 has a light trap 16 that is installed at the North Pole of the integrating sphere 10, a sample surface opening 11 that is provided near the South Pole of the integrating sphere 10 and at the same time is placed in a position so that the tangential direction of the sample 40 nearly matches the center direction of the integrating sphere 10, an incidence window 12 that is installed on the equator of the integrating sphere 10, a standard surface opening 13 that is provided near the South Pole of the integrating sphere 10 and at the same time is placed in a position so that the tangential direction of a reference white surface 50 nearly matches the center direction of the integrating sphere 10, and a photometry window 14 for measurement and a photometry window 14b for reference provided near the light trap 16 of the integrating sphere 10. In this case, a photometry window 14 for measurement is provided at a position where regular reflection light from the measurement side photometry window 14a for the sample 40 is reflected by the light trap 16, and the photometry window 14b for reference is provided at a position where regular reflection light from the reference side light receiver for the reference white surface 50 is reflected by the light trap 16.