An imaging mass spectrometer capable of reducing the dependence of the resolution of a projection image on mass is offered. Also, a method of controlling this spectrometer is offered. The imaging mass spectrometer includes: a plate on which a sample is placed; a lens system through which ions generated by irradiating the sample with laser light pass; an ion optical system for separating the ions according to flight time corresponding to mass-to-charge ratio; a detection system for measuring arrival positions and flight times of the ions passed through the ion optical system and generating an image of the sample when it is ionized; and a voltage control portion for sweeping the voltage applied to an electrode included in the lens system such that the lens effect of the lens system increases with time during a given period synchronized with the laser irradiation.