In an MS unit, both an intensity of an ion having the highest intensity among the ions originating from a compound as the target of quantitative determination and an intensity of an isotopic ion are measured. A saturation detector determines whether or not digital data produced by an A/D converter from ion-intensity signals have reached a saturation level. A data selection controller selects the ion-intensity data showing the highest intensity when the signal is not saturated or the intensity data of the isotopic ion when the saturation has occurred or is probable to occur. When the latter data is selected, an ion intensity converter converts the intensity data into values corresponding to the intensity data of the highest-intensity ion by multiplying the intensity data by a factor calculated from a known isotopic abundance ratio.