In an optical fiber sensor comprising an optical fiber coupled with a light emission element and another optical fiber coupled with a light receiving element, end portions of the optical fibers other than those coupled with the aforementioned elements being arranged side by side and joined together in a closely contacting relation with the end surfaces of the optical fibers placed in opposition to a slit member to be detected such that the arranged direction of the optical fibers is aligned with a shifting direction of the slit member; the end portions of the optical fibers are formed into a flat configuration and joined together such that the flat side surfaces of the two fibers are brought into a surface contacting relation.