PURPOSE: To make constant the ion passage band width irrespective of the mass number and obtain analytical result with good accuracy by varying the max. value (v) of high frequency AC voltage, which is impressed on a quadripolar electrode upon superposing over a DC voltage (u), in proportion to the 2/3th power of time while u/v is maintained constant.

CONSTITUTION: Ions from an ion source 2 are accelerated with a constant voltage, introduced to a quadripolar electrode 3, mass-separated, sensed by an ion sensor 6, and taken into a control processing part 10, where the mass spectrum is determined. The processing part 10 gives a voltage control signal to a scaning power supply 8 through a D/A converter 12, and in compliance with the signal a superposition of the DC voltage (u) and high frequency AC voltage v.coscot is impressed by the power supply 8 upon each electrode rod of the electrode 3, and (v) is varied in proportion to the 2/3th power of time while the ratio u/v is kept constant.