PURPOSE: To enable the simple adjustment of a pullout electrode in a short time by providing a detector and a display corresponding to respective illuminants so as to detect light from a plurlaity of the illuminants when the pullout electrode is at the predetermined position.

CONSTITUTION: A slit 10 is so set as to allow a maximum amount of light from an illuminant 9 when a pullout electrode 2a is set at the predetermined position. The light from the illuminant 9 provided in an ion source part 1 passes the slit 10 of the pullout electrode 2a and is received by a detector 11 provided at the rear of the electrode 2a. As a detected signal from the detector 11 gives a voltage change depending upon a light receiving amount, the voltage is measured by a measurement instrument 12 and the result of the measurement is shown on a display 13. By controlling the position of the pullout electrode 2a in such a way as to obtain the maximum value of measurement with each instrument 12, therefore, the position of the electrode 2a can be adjusted to the predetermined position.