A transmission system includes: a first transmission device that receives a first signal from the work path; a second transmission device, coupled to a protection path in a redundant configuration with respect to the work path, that receives a second signal from the protection path; and a first communication device coupled to the first and second transmission devices, wherein the first communication device, when detecting switching information from the first transmission device, notifies the first transmission device of first switching notification information and notifies the second transmission device of second switching notification information, the first transmission device stops relaying the first signal to the first communication device in response to the first switching notification information from the first communication device, and the second transmission device starts to relay the second signal to the first communication device in response to the second switching notification information from the first communication device.