PURPOSE: To realize an extremely high-reliability signal transmission through automatic switching from the using circuit to the stand-by circuit in case the signal reception level of the using circuit side lowers less than the level of the stand-by circuit by a fixed level along with the reception level of the stand-by circuit kept at a level higher than the fixed value.

CONSTITUTION: The transmission signals are branched into two parts at the transmission side, and these signals are transmitted at one time through the using and stand-by circuits each. On the other hand, the output levels of the channels in the using and stand-by circuits are monitored through level monitor circuits 13-1∼13-n plus 14-1∼14-n, and at the same time the comparison is given to the output levels through using/stand-by level comparators 15-1∼15-n. And in case the reception signal level of the using circuit side lowers less than the reception level of the stand- by circuit side by a fixed level along with the reception level of the stand-by circuit kept at a level higher than the fixed value, a certain channel of whole channels are switched en bloc and automatically from the using circuit to the stand-by circuit. In such way, an extremely high-reliability signal transmission circuit can be realized.