An image forming apparatus includes a charging rotary brush in contact with a photosensitive member, a laser beam irradiator to form an electrostatic latent image, a developing unit for developing the latent image into a visible image and collecting residual developer remaining on the surface of the photosensitive member after the visual image is transferred to a transfer material, and a power source unit for applying to the charging rotary brush an oscillating voltage fulfilling the requirement of: .DELTA.t 3.times.L/Vpc wherein .DELTA.t is the time taken for the oscillating voltage to vary between .+-.300 V above and below the central oscillating voltage value between the peaks of the voltage, L is the printing pitch, and Vpc is the speed of movement of the surface of the photosensitive member.