An analog/digital integrated subscriber circuit includes a power supply part supplying power with respect to a subscriber line, and a signal processing part carrying out a signal processing. The power supply part is provided with a D.C./D.C. converter which controls an ON-time of a switching by detecting an output voltage and an output current supplied to the subscriber line. The D.C./D.C. converter includes a switching mechanism for switching a power supply characteristic of the power supply part to a constant current characteristic up to a maximum output voltage when supplying the power with respect to a digital subscriber line and to a constant voltage characteristic having a voltage lower than the maximum output voltage when supplying the power with respect to an analog subscriber line.