PURPOSE: To test a self-return and a remote return at an opposite station automatically and accurately by adding an optical path switching mechanism to the optical connection part of an optical transmitter.

CONSTITUTION: The optical returning switch is driven by an electromagnet which is not shown at the optical connection part 50 to move along a rail 40 vertically to the paper surface, and also equipped with an optical switching mechanism 80 provided with a both-surface reflection mirror 45 which is formed in a 90° bent shape so as to diffract a light signal 10 in opposite right-angled directions when the switch is inserted into the optical path of the light signal 10. The reflection mirror 45 has the center part bent at right angles and is arranged where a couple of parallel light signals 10 are refracted at right angles, namely, where reflection mirror surfaces face the light signals 10 at 45° and the vertex side of bending is in the center of the interval of the parallel signals 10.