To provide a method and apparatus configuration which cover a wider range of mass dimensions by a single measurement in the MALDI ion trap type mass spectrometer.

A testpiece is irradiated with pulsed laser more than once while changing little by little a time duration between irradiation time t0 of pulsed laser in the MALDI section and period of time t1 when RF voltage is applied to a ring electrode attached in the ion trap section. Subsequently, summing or summing averaging is taken about the resultant mass spectrum obtained in this way. The shorter a time duration between t0 and t1 is, the more often the ion with smaller mass/charge ratio tends to be trapped into the ion trap space 122. A method of making the measurements a lot of times by shifting a time duration in this way thereby allows mass spectrum covering a wider range of mass dimensions to be obtained.

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