A receptacle (6) of an optical fiber collimator has an end surface (6b) supported along a plane extending at right angles to the axis of the receptacle and a contact surface (6,6') at the other end. The contact surface also extends at right angles to the axis of the receptacle. A receptacle holder (8) has a recess (8g) within which the receptacle (6) is received. Adjusting screws (24) are devised to adjust radial position of the receptacle. A lens holder (12) is attached to the other end of the receptacle holder. A slide (46) plate has a sliding surface in surface contact with the contact surface (6b') of the receptacle. The slide plate is urged toward the one end of the receptacle so as to bring the sliding surface into contact with the contact surface of the receptacle. An optical fiber (2) is inserted into the receptacle through a ferrule (4). If the optical fiber is twisted, the slide member permits ready rotation of the receptacle in a torsional direction so as to prevent the optical fiber from being subjected to torsional stress. Also, opposite ends of the receptacle are gripped in surface contact and supported in a plane extending at right angles to the axis of the receptacle holder. This arrangement prevents inclination of the receptacle and thus, optical misalignment of the optical fiber and the lens if the weight of the optical fiber is applied.