A developing device comprising: a developing roller which is adapted to carry toner and is disposed to be opposed to a photosensitive drum; a magnetic roller disposed to be opposed to the developing roller so that the magnetic brush supplies the toner to the developing roller and takes off the toner from the developing roller; a capacitor; a transformer having a primary connected to the capacitor and a secondary which is adapted to output an alternating voltage to be applied to the developing roller; and a switching portion including a control signal generating portion for generating a control signal, and a switching element connected to the capacitor so as to make and break current to a serial circuit of the capacitor and the transformer based on the control signal, characterized in that when the switching portion changes the duty ratio of the control signal step by step in a plurality of times toward a target duty ratio, after the duty ratio is changed, in a period while the voltage between electrodes of the capacitor is changing due to the duty ratio change, the switching portion is adapted to change the duty ratio of the control signal in a predetermined second time slot in which current flowing in the switching element is smaller than that flowing in a first time slot, wherein the predetermined first time slot is a time slot in which the voltage between electrodes of the capacitor is smaller than a median of a fluctuation range, the predetermined second time slot is a time slot in which the voltage between electrodes of the capacitor is the median of the fluctuation range or higher, and the period is a period of a resonance frequency determined from a capacitance of the capacitor and an inductance of the transformer. a developing roller which is adapted to carry toner and is disposed to be opposed to a photosensitive drum; a magnetic roller disposed to be opposed to the developing roller so that the magnetic brush supplies the toner to the developing roller and takes off the toner from the developing roller; a capacitor; a transformer having a primary connected to the capacitor and a secondary which is adapted to output an alternating voltage to be applied to the developing roller; and a switching portion including a control signal generating portion for generating a control signal, and a switching element connected to the capacitor so as to make and break current to a serial circuit of the capacitor and the transformer based on the control signal, characterized in that when the switching portion changes the duty ratio of the control signal step by step in a plurality of times toward a target duty ratio, after the duty ratio is changed, in a period while the voltage between electrodes of the capacitor is changing due to the duty ratio change, the switching portion is adapted to change the duty ratio of the control signal in a predetermined second time slot in which current flowing in the switching element is smaller than that flowing in a first time slot, wherein the predetermined first time slot is a time slot in which the voltage between electrodes of the capacitor is smaller than a median of a fluctuation range, the predetermined second time slot is a time slot in which the voltage between electrodes of the capacitor is the median of the fluctuation range or higher, and the period is a period of a resonance frequency determined from a capacitance of the capacitor and an inductance of the transformer.