PURPOSE: To develop sensitivity and to sharply widen a dynamic range by adjusting gain in such a manner that when ion generation quantity is higher than a threshold level the generation quantity is controlled to be lowered and when ion generation quantity is lower than a threshold level the generation quantity is controlled to be raised.

CONSTITUTION: A monitoring section 18 which monitors the generation quantity of specimen ions is added to a conventional ion detecting device, and ionizing conditions of ionizing sections 2, 4, 6 are adjusted in such a manner that when the quantity of specimen ions detected by the monitoring section 18 is higher than a threshold level the ion generation quantity is decreased and when the quantity of specimen ions is lower than a threshold level the ion generation quantity is increased. At the same time the gain of detecting, amplifying sections 14, 16a is adjusted to compensate the increase or decrease of ion generation quantity caused by the adjustment. Thereby the sensitivity is improved and the dynamic range is widened.