PURPOSE: To minimize switching time by performing logical operation among outputs of the 1st detecting means detecting the state of signal of an active transmission line, of the 2nd detecting means detecting the state of signal of a spare transmission line, and an output obtained by delaying the output of the 1st detecting means to switch and control the active line and the spare line.

CONSTITUTION: When the active transmission line N and the spare transmission line E are normal, an output of detecting circuit 1, 2 is both logical 1 and a contact rl of a relay RL is thrown to the N position. When a signal of the N is interrupted at a point of time X from this state, an output of the detecting circuit goes to 0 and is inputted to an S terminal of an FF6 so as to turn off the RL and throw the contact rl to the E position. Further, the output of the detecting circuit 1 is delayed by T3 at a delay circuit 3 and inputted to an inverter 4 and its output (d) is inputted to one input terminal of an AND gate 5. An output of the detecting circuit is inputted to the other input terminal of the AND gate 5 and then an output (e) of the AND gate 5 is inputted to a terminal R of the FF6. Thus, the line is switched to the N at the point of time Y.