PURPOSE: To remarkably reduce the production of the output signals, even if an ion detector is vibrated, by making an ion collector and a shield case at the same potential.

CONSTITUTION: An electrically insulating member 15 mechanically fixes a sheild case 2 to the end case of an analyzing tube 1, while electrically insulating the shield case 2, and a shiled case electrode 16 for guiding the potential of the case 2 to the outside extends through a portion of the insulating member 15. An ion collector electrode 11 is connected with the irreversible input terminal of an operational amplifier 14, the shieled case electrode 16 is connected with the output terminal of the amplifier 14, and the output terminal of the amplifier 14 is connected with the reversible input terminal and further with the input side of an amplifier 12. If the amplification of the amplifier 14 is infinite, it is considered that the output of an ion collector 3, i.e., the potential of an ion collector electrode 11 is equal to that of the electrode 16. Thus, the output of the ion collector 3 is neutralized with the floating capacity which is varied by the vibrations with the shield case 2.