PURPOSE: To suppress the breakage of ICs and heat generation of a built-in power source circuit, which occur when a supply voltage is high, by installing a voltage detecting circuit which detects an input voltage to the inside of a multifunctional telephone set and, when the input voltage is higher than a specified value, lowering the output voltage value of a power supplying source in an exchange device.

CONSTITUTION: When the power supply to an exchange system is turned on, a transistor Q5 is turned on from the CPU 3 of an exchange device A and, as a result, a voltage of 48V is impressed from a power source BAT to terminals D1 and D2 of a multifunctional telephone set B. The impressed voltage is compared with a reference voltage 22V by a comparator circuit 12 and the compared result is outputted to a CPU 11 informing that the impressed voltage is higher than the reference voltage. Then the CPU 3 turns off the transistor Q5 and turns on another transistor Q2. When a voltage of 20V is impressed upon the terminals D1 and D2 of the multifunctional telephone set B and, as a result, the input of the comparator circuit 12 becomes lower than the reference voltage of 22V. Therefore, output to the CPU 11 is stopped and this condition is maintained.