Abstract: A constant voltage power supply circuit includes oscillation means; a step-up or step-down circuit including coil means and capacitive means; detecting means that detects an outputted voltage value of the step-up or step-down circuit, and compares the detected value with a predetermined reference value to output a detected information, at which either the detected value exceeds the reference value or becomes lower than the reference value; and control means that controls, based on the detected information of the detecting means, supply of the pulse signals outputted from the oscillation means to the step-up or step-down circuit.