PURPOSE: To stop the transmission of defective data in early stages, and to reduce an influence exerted on other devices by stopping the light emitting operation of a light emitting unit, and uncoupling optically the light emitting unit from an optical network when the power level of a branched optical signal falls below a prescribed level.

CONSTITUTION: When the power level of the optical signal sent from the light emitting unit 4 to a transmission trunk line 2 becomes lower than the prescribed level because of the secular change of a light emitting element or the like, and the possibility of transmitting the defective data arises, this state is detected by a optical level detecting part 8. The detecting part 8 stops the light emitting operation of the light emitting unit 4 in response to this detection output, and simultaneously, it controls an optical switch 3 so as to uncouple optically the light emitting unit 4 and a light receiving unit 5 from the transmission trunk line 2. Thus, in the case of the occurrence of the defective data, the transmission of the defective data is stopped in the early stage so that the influence exerted on other devices on the optical network can be made small.