By altering a developing bias, a plurality of patch images are formed on a photoconductor, and a density of each patch image is measured, thereby judging whether or not a target density value is within a range of the plurality of measured density values. If the target density value is outside the range, a developing bias which realizes the target density is obtained by an interpolation, in accordance with two measured density values that are closest to the target density value among a plurality of measured densities, and with a developing bias of two patch images that are related to these two measured density values.