An endoscope includes a generally cylindrical optical waveguide, an light source emitting a laser beam, and a generally annular photodiode. The optical waveguide is provided at the distal end of the endoscope. The light source is on or near the center axis of the optical waveguide for emitting a laser beam onto a subject, so that light reflected from the subject enters an annular front-end surface of the optical waveguide. The photodiode faces an annular rear-end surface of the optical waveguide. A waterproof covering tube covers the optical waveguide and the photodiode.