In an optical transmission system, a first monitoring section configured to monitor a first optical signal on a first optical transmission line to detect a first failure. A modulating section generates a second optical signal obtained through modulation of a signal corresponding to the first optical signal based on the first failure, to transmit the second optical signal onto a second optical transmission line which is connected with the first optical transmission line and a terminal unit. A second monitoring section monitors the second optical signal on the second optical transmission line to detect a second failure, and a detecting section detects the first failure from the second optical signal on the second optical transmission line. A line switch configured to selectively switch an optical transmission line to be connected with the terminal unit, from the second optical transmission line to another optical transmission line based on the first failure detected by the detecting section and the second failure detected by the second monitoring section.