A discomfort index, S, is calculated with an equation, S=0.3135.times.(Loudness value)+3.4824.times.(Tonality value)-3.1460. This equation uses a loudness value and a tonality value, both psychoacoustic parameters obtained from a sound from the image forming apparatus at a location 1.00 m.+-.0.03 m apart from an end of the image forming apparatus. A sound caused at the time of charging an image carrier is improved so that the discomfort index S satisfies S<-0.5.