PURPOSE: To attain an inexpensive switching circuit with simple constitution by switching a signal while neglecting a little fluctuation of a propagation delay time when the signal to be switched is a low frequency and the width of one bit is large.

CONSTITUTION: An input signal of each system is converted by a bipolar/unipolar converter B-U and then branched into two; the one is transmitted as an active signal normally via a transmission logical section T.LOG and the other is collected to a spare transmission period changeover switch T.SYNC.SW, which outputs an output of a transmission logical section T.LOG of a spare system #SP as a spare signal in the normal state. A frame synchronizing circuit F. SYNC performs frame synchronism in each system at the reception side and a propagation delay time adjusting circuit DL.ADJ adjusts the time difference in the system to a sufficiently small value. The input signal is inputted to a serial/ parallel converting circuit at a reception synchronism changeover switch R. SYNC.SW' and restored by a parallel/serial converting circuit then the result enters the reception logical section R.LOG.