A fiber retaining structure is disclosed for readily and reliably mounting and retaining optical fibers on a casing in which a light-emitting element and a light-receiving element are incorporated. The fiber retaining structure includes a first link 61 for moving both a second link 26 for crashing O-rings with optical fibers 2 and 3 inserted therethrough against a casing 24 in which a light-emitting element 6 and a light-receiving element 7 are incorporated and holders 27 and 28 for holding the optical fibers when a lever 29 is pushed down, and a spring material. Since the inner diameter of a sealing material for sealing a gap around the optical fiber is set substantially equal to the outer diameter of the optical fiber and since the holding of the optical fibers by the entire holders is performed separately, the distal ends of the optical fibers can be readily and reliably brought into abutment with the light-emitting element and the light-receiving element and the optical fibers can be reliably retained.