To reduce the manhours and cost when an add or drop circuit is concretely developed by selecting one of both signals which are branched out from an inputted transmission signal or the signal which is outputted from a time slot conversion means in response to an external selection signal.

A signal B which is branched out to a connecting device can be outputted after a time slot replacement circuit is set and therefore a selector part 21D is set to always select the signal B. This is equivalent to a fact that the signal B is directly inputted to a time slot conversion part 22D. Then the signal B is outputted to the connecting device after the part 22D has performed a time slot replacing operation. It's desired that the signal B sent to an add circuit 20A is outputted with no processing. Thus, a selector part 23D is set in a through state to transmit the signal B as it is. As a result, the signal B is outputted to the circuit 20A.