pet system similarly to the conventional pet system shown in fig5 a tunnel 2 is formed in a gantry 1 a subject is laid on a top board 4 of an examining table 3 and the top board 4 with the subject is inserted into the tunnel 2 however the gantry 1 has a novel structure the structure is as follows in this pet system the gantry is largely extended in a direction of the body axis of the subject the gantry l is cylindrical and extends along body axis as shown in fig2 and fig3 the gantry 1 is divided into a plurality of units 5 in the direction of the body axis each of the units 5 is movable in a direction perpendicular to the body axis for example a guide is arranged below the gantry 1 and each of the units 5 is supported and guided by the guide each of the units 5 can be slid along the guide and thus moved in directions horizontal and vertical to the body axis the unit 5 may be moved by a driving mechanism as shown in fig4 a large number of detectors 6 are incorporated in each of the units 5 in such a way that the detectors 6 are aligned in directions of a circumference of the unit 5 and the body axis the detectors 6 are connected between the units through cables according to this embodiment.