Image forming apparatus and charging bias adjusting method therefor

An image forming apparatus includes a latent image bearer to rotate, an image forming unit including a charger and a developing device, a charge power supply to output a charging bias applied to a charger, a transfer device, a toner adhesion amount, and a controller. The controller causes the image forming unit to form a background fog pattern in a background area of the latent image bearer while changing a background potential, acquires toner adhesion amount values detected at different positions of the background fog pattern, having different potentials, sorts the toner adhesion amount values in an order of the background potential, determines a relation between the background potential and background fog amount based the toner adhesion amount values except any toner adhesion amount value out of monotonicity, and adjust the charging bias to an optimum value computed based on the determined relation.