@ This invention provides a line protection switching system of which reliability is improved, and the time for protection line switching is decreased. In order to increase the reliability, the service signals are sent in parallel via plurality of transmission lines (11,12 ---), using a service bit area of PCM bit frames. Unlike the prior art system, one embodiment ofthe present system switches a failed transmission line to a stand by line (1n), at a station where it detected a fault before waiting for switching of the opposite station. At the same time the switching command is sent to the opposite station. If the switching of the opposite station fails, the station commands retry of the switching. But if it fails again, the station switches the transmission line to the initial state. Since a low priority signal is transmitted through the stand by line (1n), the total system is economized.