PURPOSE: To surely prevent the light reflection on the incident side by a simple method by disposing a transparent plate or optical fiber in the light beam west position on the incident side of a light isolator and bonding and fixing both by means of an adhesive agent of roughly equal refractive indexes.

CONSTITUTION: A light isolator 10 is provided with circular cylindrical columns 12, 12' in such a manner that parallel beams 11 pass therein. Sufficient nonreflection treatment is applied between the isolator 10 and the lenses 12, 12'. A glass plate 18 of about 1mm thickness is adhered by means of an epoxy resin 19 to the front face of the incident side lens 12. If the refractive indexes of the glass plate 18, resin 19 and lens 12 are selected roughly equal, the reflection at the beam west 13 may be made 0W0.01. On the other hand, the reflected light on the front face of the glass plate 18 is deviated largely from the beam west position and therefore it hardly returns to a semiconductor laser 14 by passing through a coupling lens 16. Even when an optical fiber 20 is adhered by a resin 22 in place of the glass plate 18, the light reflection on the incident side may be surely prevented in a similar manner.