To automatically set an appropriate detector voltage combining an accuracy of counting with a long life of a detector even when a plateau region is not clear in a relation between the voltage of the detector and the counted value in the pulse count type detector.

While gradually increasing the detector voltage from a low voltage, the counted value of a pulse signal generated by binarizing an output signal from a secondary electron multiplication tube is sequentially obtained at each voltage (S1, S2). A difference value and a second-order difference value of the counted value are calculated each time the counted value is obtained (S3, S4), and it is regarded as a semi-plateau region when the second-order difference value indicating a change of an increasing rate of the counted value accompanying an increase in the voltage is zero or less (S5, S6). After the measurement, the maximum counted value in the semi-plateau region is assumed to be the true counted value, and the minimum detector voltage within an acceptable range of the counted value according to sensitivity is determined to be the appropriate voltage (S9, S10).