An illumination light application structure provided for irradiating an observation object with illumination light in an observation optical apparatus provided with an objective optical system inside a lens barrel whose top is configured into a slender shape, includes a wavelength converting element located in the proximity of a position of an entrance pupil of the objective optical system a light source emitting light whose wavelength is converted by the wavelength converting element and an irradiation device irradiating the wavelength converting element with the light emitted from the light source through the objective optical system.
