PURPOSE: To provide an optical distribution system which can be easily extended with reduction of the cost and the occupying space.

CONSTITUTION: An optical distribution system consists of a stand-by transmission device 12, a 1st n-branch photocoupler assembly 48 where each branch port is connected to a stand-by electric-optic conversion module 8 of a working transmission device 2 and a common port is connected to a 1st optic-electric conversion module 20 of the device 12, a 2nd n-branch photocoupler assembly 50 where each branch port is connected to a stand-by optic-electric conversion module 10 of the device 2 and a common port is connected to a 1st electric-optic conversion module 18 of the device 12, a 1st optical attenuator 28 which is provided between the common port of the assembly 48 and the module 20, and a 2nd optical attenuator 28 which is provided between the common port of the assembly 50 and the module 18.