PURPOSE: To enable output transmission by one cable instead of two coaxial cables for transmitting output of two high-low wave height discriminating circuits provided in a detector in order to stabilize the gain of a photomultiplier in a scintillation counter tube for detecting γ-rays.

CONSTITUTION: Two wave height discriminating circuits 4, 5 of a high level and a low level are provided in relation to the output pulse of a photomultiplier 2, and a delay circuit 8 is provided in relation to the output of the high level wave height discriminating circuit so as to output later by the fixed time than the output of the low level wave height discriminating circuit 5. Also with the change of pulse height, wave height discriminating circuits 10, 11 on the converter side count the pulse output by separating it into pulses having passed the high and low level wave height discriminating circuits 4, 5 of a detector. Since two kinds of signals can be transmitted by one coaxial cable, two coaxial cables required heretofore can be made one.