A quadruple mass spectrometer capable of reducing a settling time-period necessary in a process of changing, in a pulsed or step-like pattern, a voltage to be applied to a quadruple mass filter in a scan or SIM measurement. In the SIM measurement, an optimal settling-time calculation sub-section sets a length of the settling time-period according to a difference ΔM between a next-measurement mass value and a mass value used in an adjacent measurement, and the next-measurement mass value. This makes it possible to shorten a duration of a repetitive cycle in the SIM measurement or increase a time-period assignable to a measurement operation, while ensuring a voltage stabilization time-period sufficient to detect ions having the next-measurement mass value.