In a TOFMS measurement unit, an ion acceleration section accelerates ions to send them into a flight space, within which a flight-field creation section creates, an electric field for causing ions to fly. A controller unit operates the measurement unit so as to repeat a measurement for a predetermined sample while varying a voltage applied to an electrode in the measurement unit, and calculates mass-resolving power based on each measurement result. An approximate function calculator unit finds an approximate function representing a relationship between the voltage and the mass-resolving power, based on data of combinations of the voltage and the mass-resolving power obtained under the control of the controller unit. A voltage determiner unit determines a voltage value corresponding to a target value of the mass-resolving power by the approximate function, and determines the voltage value as a voltage to be applied to the electrode in the TOFMS concerned.