A printer 20 includes a photosensitive drum 1, a charge roller 2 for electrically charging the photosensitive drum 1, a developing device 4 for developing an electrostatic latent image on the photosensitive drum 1 into a toner image, a transfer roller 5 for transferring the toner image onto a recording material, and a transfer residual toner charging member 8 for electrically charging toner remaining on the photosensitive drum 1. In the printer, a DC voltage substantially identical to a potential on the photosensitive drum 1 before the photosensitive drum 1 reaches the charge roller 2 is applied to the charge roller 2 together with application of a voltage of an identical polarity to a charge polarity of toner to the transfer residual toner charging member 8 during a current measuring operation for measuring a value of AC current passing through the charge roller 2, in order to set a condition of a voltage to be applied to the charge roller 2 during image formation, by applying an AC voltage to the charge roller 2.