A device for selecting a normal circuit in a communication system includes at least one pair of general function circuit modules, one pair of control function circuit modules, one pair of power supply modules, and a separate processor for controlling the device for selecting a normal circuit. Of the one pair of the control function circuit modules, the attempt for interchanging states of the control function circuit module in an active state and the control function circuit module in a standby state is made possible from the control function circuit module in the active state without fail. For stabilization of an output, the control function circuit module in the active state is switched to the standby state after the control function circuit module in the standby state is switched to the active state by the selecting circuit.