To reduce a sample consumption while improving a throughput by efficiently searching samples for MALDI for a sweet spot suitable for measurement.

Measurement points are set stating at a center point O of a well 1a formed on a sample plate as a starting point P so that a spiral scanning track Q is drawn outward from the starting point P. Preliminary measurements at the respective measurement points on the sample S are made to acquire index values of peak intensity, S/N, etc. When a state in which an index value is less than a specified value repeats a predetermined number of times, it is determined that laser light irradiation deviates from the sample S and the measuring operation is quit. Then it is determined that a measurement point where a maximum index value is obtained is a sweet spot, and a plurality of measurements are made at the measurement point to acquire a mass-spectrum of the sample S. Consequently, while unnecessary preliminary measuring operation in a state of deviation from the sample S is reduced, the sweet spot is accurately found.