Abstract: A fiberscope for conducting observations in a region ordinarily filled with an opaque liquid, particularly, inside blood vessels or the heart. A recess is provided at a front end of a cover tube for accommodating a front end portion of an image fiber and a bundle of illuminating light transmitting optical fibers which are bonded to a light guide front end part molded to conform to a part of the inner wall of a cover tube, and an optical fiber bundle provided by bonding front end portions of the image fiber and the light guide is fixedly bonded to the inner wall of the cover tube. A transparent liquid, physiological saline in preferred embodiments, is injected from a hole near the end of the fiberscope at a flow rate determined in accordance with the flow rate of the opaque liquid to temporarily displace some of the opaque liquid to permit observation. Further, the injection of the transparent liquid may be done in synchronization with variations of the pressure and/or flow velocity of the opaque liquid.