An illumination light detecting optical system includes an element emitting excitation light; an insertion part of a long-narrow-shape; a fluorescence luminescence component arranged near a top of the insertion part; an excitation light guiding element which leads the excitation light emitted from the light emitting element to the fluorescence luminescence component; and a light detecting element which detects return light which is a part of the illumination light. The optical system is also provided with a pillar-shaped transparent component for irradiating illumination light arranged coaxially at a forward position of the fluorescence luminescence component; an illuminating-light-extracting-element for extracting the illumination light from the pillar-shaped transparent component, out of the illumination light which enters into the inside of the pillar-shaped transparent component; and a deflecting element by which the extracted illumination light is directed toward the light detecting element.