An image forming includes an apparatus main body, a photoreceptor drum, a charging apparatus, a developing device, a transfer apparatus, a charging bias applying unit, a developing bias applying unit, a bias adjusting unit, and a print density measurement unit. The bias adjusting unit forms a non-charged area on a circumference surface of the photoreceptor drum and applies a developing bias constituted of a first electric potential to a developing roller to form a first toner image by an electric potential difference between the non-charged area and the developing roller. The charging bias applying unit forms a second toner image by an electric potential difference between the electric potential area and the developing roller. The charging bias applying unit decides a value of a charging bias corresponding to a target electric potential from measurement results of print densities of the first toner image and the second toner image.