To provide a device for preventing damages due to an AC short circuit, that can prevent a damage due to a short circuit of an AC power supply line to a subscriber line and can be restored automatically, when the AC short circuit is recovered.

At least one of current detection circuits 3, 4 detects an AC short circuit current, a changeover control circuit 11 turns off an optical switch 12. When both voltage detection circuits 5, 6, which is able to detect the presence of an AC short circuit, no longer detect AC short circuit voltage the changeover control circuit 11 will turn on the optical switch 12, even if a connection between a subscriber and an interface circuit 13 is interrupted.