As a control parameter given to a direct-current (DC) voltage generator which generates a DC voltage for ion selection, a “mass-related offset” for allowing an adjustment of the offset for each mass-to-charge ratio is provided in addition to the “gain” and “common offset” which respectively determine the gradient and position of a scan line drawn on a stability diagram during a mass-scan operation. In an automatic adjustment operation using a standard sample, under the control of an automatic regulator, the “gain” and “common offset” are initially set, after which the “mass-related offset” for each mass-to-charge ratio is determined so that the mass-resolving power will be substantially uniform, and these data are stored in a control data memory. In an analysis of a sample of interest, a quadrupole voltage controller controls the DC voltage generator and a radio-frequency (RF) voltage generator according to the control parameters read from the memory.