A sample S is irradiated with a two-dimensionally spread ray of laser light to simultaneously ionize substances within a two-dimensional area on the sample. The resultant ions are mass-separated by a TOF mass separator 4 without changing the interrelationship of the emission points of the ions. The separated ions are then directed to a two-dimensional detector section 7 through a deflection electric field created by deflection electrodes 61 and 62. The two-dimensional detector section 7 consists of a plurality of detection units 7a arranged in parallel, each unit including an MCP 8a, fluorescent plate 9a and two-dimensional array detector 10a. The magnitude of deflecting the flight path of the ions by the deflection electric field is changed in a stepwise manner with the lapse of time from the generation of the ions so that a plurality of mass analysis images are sequentially projected on each detection unit 7. When the mass analysis image shifts from one detection unit to another, the data acquisition operation by the two-dimensional array detector in the previous detection unit is discontinued. As a result, a predetermined number of the latest images are held inside the detector. Thus, the measurement time can be extended to widen the measurable mass-to-charge ratio range, while ensuring a high mass resolution.