PURPOSE: To secure an accurate sensitivity correction for the detection system through a simple constitution for the test device which obtains the tomogram iamge of the subject via the transmissible radiant rays by providing the transmission material for correction plus the detaching mechanism.

CONSTITUTION: Radiation source 1 irradiates the fan-shaped radiant rays to subject 3, and detector 6 contains plural units of photo detector 5 to detect each part of fan- shaped beam 2 individually which passed through subject 3. In addition, the following units are added to constitute a test device: the means which turns detector 6 and source 1 in order to turn the radiant rays within the fan-shaped surface and thus to cut subject 3; and A/D converter 11 which recompose the beam data obtained through detector 6 in the form of the picture regarding the absorption destribution of the radiant rays at the cut-off part. Then wedge 7 is inserted into beam 2 along with radiation transmission material 8 and 9 inserted detachably into beam 2 in order to minimize the test width of the readings of detector 6. Thus, the data of material 8 and 9 are collected before collection of the optical data and before or after collection of the data of subject 3, thus obtaining the desired corrected data for each collected data.