A power source circuit consisting of a combination of a constant-voltage circuit (1) and a voltage raising/lowering circuit (2) which produces a plurality of output voltages by raising or lowering the output of the constant-voltage circuit. When a change in the power source voltage or the presence or absence of a heavy load is detected that information is supplied as a control mode signal to both the circuit (1) and circuit (2). For instance, when the power source voltage is reduced or when a heavy load is driven, the output voltage of the circuit (1) lowered and instead the rate of change in the output voltage of the circuit (2) is adjusted so that the output voltage may eventually become the same as in steady state.