PURPOSE: To decrease the number of radiation detectors, by providing a parallel slit type colimator which rotates on the face on which a radiation detector is arrayed, and reconstituting a plane-like distribution image of RI from a slit array direction distribution data of a counting value of each rotation angle.

CONSTITUTION: A lot of radiation detectors 12 are arrayed on a concentric ring, and a rotary parallel slit type collimator 13 having a lot of linear slits S1, S2... is placed on a substrate 11. In case when an output is generated from the i-th detector 12, it is decided that a radiation is made incident through a slit Sk, and a signal showing the slit Sk is derived from the number (i) and a rotation angle θj of the collimator 13. The radiation which is made incident through each slit S1, S2... in case when the rotation angle is θj is counted at each slit, and a distribution data (θj) in the slit array direction of the counting value is obtained. The data is sent to an operating device, an operation for reconstituting an original picture from a prjection data of convolution, back projection, etc. is executed, and a plane-like distribution image of RI is reconstituted and displayed.