1. A fluorescent image obtaining apparatus comprising an illuminating means that guides an excitation light to a measurement area and illuminates the measurement area with the excitation light, an image obtaining means that obtains a fluorescent image with an image obtaining element based on the fluorescent light emitted from the measurement area upon the illumination thereof by the excitation light and which has been passed through an imaging optical system, and a readout means that reads out an image signal based on the fluorescent image obtained by the image obtaining means, further comprising a statistical quantity computing means that computes a statistical quantity based on the image signal of apredetermined region of the fluorescent image obtained by the image obtaining means, wherein the image obtaining means is provided with a magnification control means that determines the size of the fluorescent image by controlling, based on the statistical quantity, the magnification rate of the imaging optical system.