An image forming apparatus includes a rotatable photosensitive member chargeable by a charging member; an applying device for applying to the charging member a charging bias; an image forming device for forming a toner image on the photosensitive member; a setting device for setting a first frequency of a first charging bias in a first rotation speed mode and for setting a second frequency of a second charging bias in a second rotation speed mode; a detector for detecting current passing between the charging and the photosensitive members during application of a test bias to the charging member; and an adjusting device for adjusting the second charging bias based on an output of the detector when the photosensitive member is rotated at a first speed and a test bias having the second frequency is applied to the charging member when switching from the first to the second rotation speed modes.