PURPOSE: To enable exact execution of energy calibration of a detector by executing the energy calibration by using a gamma-ray source in a detector provided with a semiconductor detecting element for charged particles which has a large area.

CONSTITUTION: A detecting element for energy calibration is provided separately, and in this case, a semiconductor detecting element 24 for gamma rays and a bias power source circuit 26 for calibration which gives a bias voltage to the semiconductor detecting element 24 for gamma rays are provided. Besides, a switching circuit 28 composed of switches 28a and 28b is provided, and the switch 28a is turned ON to connect a semiconductor detecting element 10 of large area on the occasion of ordinary measurement of charged particles, while the switch 28b is turned ON to connect the semiconductor detecting element 24 for gamma rays on the occasion of execution of the energy calibration. In the case when the energy calibration is executed, the energy calibration of a detector using the semiconductor detecting element 10 of large area can be executed by means of a factor of proportionality.