To provide a power source switchable subscriber circuit in which devices are packaged at high density and noises caused by fluttering at the time of power source switching are reduced.

In this power source switchable subscriber circuit, a power transistor is shared, discrimination circuits 116 and 117 for discriminating power source switching are provided with a hysterisis and further, a power source switching spot is not uniquely determined by the voltage of first and third power sources 103 and 111 but can be set as desired by supervisory circuits 114 and 115 and the discrimination circuit 116 and 117.