The invention relates to a power source circuit in which an output of a power source battery is converted by a voltage regulator constructed by a predetermined voltage regulating system and is output, an output voltage is divided by a resistance controller whose voltage dividing resistance value can be discontinuously adjusted, the divided voltage is input to the voltage regulator, and thereby controlling an output voltage, wherein means for controlling a voltage changing speed of a resistor is connected in parallel with a predetermined resistance element of the resistance controller. With the above construction, a voltage drop and a deterioration of voltage regulating efficiency which easily occur in the case of changing the output voltage from a low value to a high value in a stepwise manner are reduced. Then, the life of battery is prolonged and, further, an erroneous warning of the battery life is prevented.