An optical CT apparatus (10) is mainly composed of a container (12) filled with an optical interface member (20) a light-projecting section, comprising a light source (22) and an optical switch (24), for projecting light into the container (12) a light-detecting section, comprising photodetectors (30) and shutters (32), for detecting the light from the inside of the container (12) and an arithmetic/control section (14) for calculating a spatial distribution of absorption coefficient. The arithmetic/control section (14) has a function of determining a spatial distribution of a characteristic amount concerning a part of an object to be measured (200) according to a comparison of an optical intensity signal actually measured by each detector (30) in a state where the container (12) is filled with the optical interface member (20) with an optical signal actually measured by each detector (30) in a state where the optical interface member (20) is partly replaced by the part of object to be measured (200).The apparatus (10) further comprises deformable light-shielding means (18) provided at the edge of an opening portion of said container for blocking light from the outside.
