To provide an illuminating method realizing efficient and accurate observation when it is used as the light source of a microscope by almost eliminating the loss and the interference of light while efficiently irradiating only light in the neighborhood of a desired wavelength through the use of a laser light source.

The light of a laser light source 1 is condensed by the first condensing lens 2, the condensed light is made incident on one end of a multimode fiber or a large-diameter optical fiber 3 and exits from the other end. The light is then condensed by the second condensing lens 4 to be irradiated on an object 5. Further, oscillation of the light source 1 is made multimodal to extend the wavelength region, or a transmission distance is varied with time to further reduce interference.