PURPOSE: To enhance the measuring accuracy of transmissivity in a sample chamber for measuring transmissivity by preventing the incidence of stray light caused by the reflection of the sample other than sample transmitted light and reference light on a luminous intensity detector.

CONSTITUTION: A partition wall 13 having a through-hole at a position permitting only the light 8 transmitted through a sample 7 to pass and optically isolating the space containing the sample and a space containing an integrating sphere (luminous intensity detector) 9 is provided in a sample chamber 12. By this constitution, the scattered light (so-called stray light) 11 generated when the reflected light 10 from the sample 7 impinges against the inner wall surface of the sample chamber 12 from being incident on the luminous intensity detector 9 to enhance the measuring accuracy of transmissivity.