A method of setting a laser-light intensity value includes: emitting laser light, the laser light being excitation light, a fluorescent-dyed biological sample being irradiated with the excitation light and emitting light; detecting fluorescence emitted by the biological sample, and outputting a signal corresponding to a brightness value; prestoring relation information, the relation information including the plurality of laser-light intensity values, and information on at least one possible correlation between a phototoxicity degree and the brightness value in relation to each of the laser-light intensity values, the phototoxicity to the biological sample resulting from the laser light; generating a fluorescence image having the brightness value based on the output signal; calculating a brightness value representative of a ROI area based on the generated fluorescence image; and referring to the relation information, and determining a laser-light intensity value satisfying tolerance of the phototoxicity based on the calculated representative brightness value.