An image forming apparatus comprising: a photo conductor drum including a photosensitive surface having a cylindrical shape, an exposure unit to form an electrostatic latent image on the photosensitive surface of the rotating photoconductor drum, a developing device for developing the formed electrostatic latent image with toner under a predetermined developing bias, a patch forming unit to form a plurality of test patches at an interval d, by developing electrostatic latent images formed on the photosensitive surface, by the developing device under a plurality of different developing biases, the interval d is {(2n-1)/2} times of a value L, the value L is a length of the photoconductor drum in a circumferential direction, a patch detect unit to detect adhesion amounts of toner of the plurality of test patches formed by the patch forming unit, and an approximate equation calculate unit to calculate an approximate equation which approximates relationship between developing bias used for forming the plurality of test patches and an adhesion amount of toner of the plurality of test patches, and further based on a midpoint coordinate point, the midpoint coordinate point being between a coordinate point (V i, M i) and a coordinate point (V i+1, M i+1), where V i indicates the i-th lowest developing bias (i is a natural number from 1 to (m-1)) among the m developing biases, and M i indicates the adhesion amount of toner corresponding to V i, and a controlling unit to calculate a value of the developing bias corresponding to a target adhesion amount by using the calculated approximate equation, and form an image by using the calculated value of the developing bias. a photo conductor drum including a photosensitive surface having a cylindrical shape, an exposure unit to form an electrostatic latent image on the photosensitive surface of the rotating photoconductor drum, a developing device for developing the formed electrostatic latent image with toner under a predetermined developing bias, a patch forming unit to form a plurality of test patches at an interval d, by developing electrostatic latent images formed on the photosensitive surface, by the developing device under a plurality of different developing biases, the interval d is {(2n-1)/2} times of a value L, the value L is a length of the photoconductor drum in a circumferential direction, a patch detect unit to detect adhesion amounts of toner of the plurality of test patches formed by the patch forming unit, and an approximate equation calculate unit to calculate an approximate equation which approximates relationship between developing bias used for forming the plurality of test patches and an adhesion amount of toner of the plurality of test patches, and further based on a midpoint coordinate point, the midpoint coordinate point being between a coordinate point (V i, M i) and a coordinate point (V i+1, M i+1), where V i indicates the i-th lowest developing bias (i is a natural number from 1 to (m-1)) among the m developing biases, and M i indicates the adhesion amount of toner corresponding to V i, and a controlling unit to calculate a value of the developing bias corresponding to a target adhesion amount by using the calculated approximate equation, and form an image by using the calculated value of the developing bias.