Technique for adjusting development voltage in developing device provided in image forming apparatus

An image forming apparatus controls a power supply circuit by supplying a control signal to the power supply circuit, and detects an electrical characteristic, e.g. an electrostatic capacitance generated between an image carrier and a developing member or a current caused to run by applying a development voltage to the developing member. The apparatus determines a change pattern of a duty ratio of a PWM signal including the control signal on the basis of the electrical characteristic. The apparatus changes the duty ratio as time passes according to the determined change pattern and outputs the control signal to the power supply circuit.