PURPOSE: To attain bidirectional transmission without making the setting of arrangement and space difficult or increasing the space in the case of unidirectional transmission by providing a light transmitter-receiver having an integrated structure using a mirror and sharing an optical lens system on both mobile object and ground.

CONSTITUTION: The information from ground is processed by a light transmission section 27 of a light transmitter-receiver on ground, sent as an optical signal 31 from a light emitting diode 11, transmits through a half mirror 29, is collimated by an optical lens system 30 and sent to space. Then the collimated light is collected by an optical lens system 30 of an on-vehicle light transmitter- receiver 100b, reflected in the half mirror 29, made incident on a photodetector 14, processed by a light receiving section 28 and various information is outputted from a video output terminal 23 or the like. Since the optical lens system 30 in common to the transmitted/received light is used to transmit/receive the light, the similar condition is enough in the arrangement to that in the case of unidirectional transmission in spite of the bidirectional transmission and the installation space is decreased.