An extension line of a tube axis of a photomultiplier tube is shifted from the center of gravity position of a front surface of a housing, which allows a space to be formed at an opposite side in the housing. A signal processing board is arranged in this space, and a high-voltage generating circuit board is fixed on the extension line of the tube axis, and thus the interior space of a housing can be effectively used. Since the signal processing board can be made adjacent to a tube wall of the photomultiplier tube, even when the length in a tube axis direction of the signal processing board is long, it becomes possible to house the same in the housing. Therefore, it becomes possible to achieve miniaturization.