A fluorescence observing apparatus including a light source for emitting excitation light, an excitation light irradiation section for irradiating the excitation light to a sample, and a fluorescence measurement section for measuring fluorescence emitted from the sample by the irradiation of the excitation light. The temperature of the light source is always sensed by a thermistor, and a Peltier element disposed in direct contact with the light source is cooled by a control section so that the temperature of the light source is maintained at 20° C. or less.