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.