A fluorescence-spectrum-recording unit, a fluorescence-image-acquiring unit, and a fluorescent-dye-density-calculating unit are comprised, the calculating unit calculates the densities D1 to Dm of fluorescent dyes 1 to m in each of all the pixels and in all of the pixels with the following equation, and, when there exists a pixel in which one of the calculation values of these densities is smaller than 0, a set value larger than the calculation value smaller than 0 is substituted for the density the calculation value of which is smaller than 0 in the equation, and the densities of the other fluorescent dyes are recalculated, relative to the pixel:
