A scanner includes a plurality of laser stimulating ray sources, a sample stage on which a sample containing a labeling substance is to be placed, a scanning mechanism, a photomultiplier for photoelectrically detecting light released from the labeling substance and producing analog image data, an A/D converter for converting the analog image data to digital image data, and a pixel density signal intensity simulating section for effecting simulation based on pre-scan digital image data produced by setting a voltage value of the photomultiplier to a given photomultiplier voltage value G0 to simulate density signal intensity of each pixel of digital image data that would be produced by setting the photomultiplier to a voltage value G different from the voltage value G0. According to the thus constituted scanner, it is possible to determine the voltage value of the photomultiplier simply and rapidly without causing on the degradation of a sample.