PURPOSE: To enable optical communication with good condition by emitting a beacon beam through a light passing area provided on a sub-mirror, thereby effectively emitting the beacon beam with high intensity and making the distribution of light intensity to the opposite party of communication uniform.

CONSTITUTION: A beacon beam from the opposite party communication is successively reflected and converged by a main mirror 1 and a sub-mirror 2, reflected by a beam splitter 4 through a collimator lens 3 and a pointing means 7 and converged on a beacon beam receiving part 17 by means of a convex lens 5. The light beam converged by the convex lens 5 is reflected by the beam splitter 4, converged on a position S by means of the collimator lens 3 through the pointing means 7, passes through a through hole 2b provided on the sub- mirror 2 and emitted to the opposite party of communication. By calculating the diameter of the through hole 2b opened on the center of the optical axis of the sub-mirror 2 from the diameter of the beacon beam flux, a distance from a final lens surface to the sub-mirror and a distance from the final lens surface to the converged position of the beacon beam, the uniformly spread beacon beam is effiectively transmitted.