In an image density correction method in accordance with an embodiment of the present invention, an image density correction is carried out by obtaining an actual toner amount through obtaining a density of a between-sheet patch image having been formed during continuous printing, reconstructing relational characteristics in which the actual toner amount is associated with each variation in developing biases, by considering that the actual toner amount and a reference toner amount are to be made in agreement, when a determination that the actual toner amount disagrees with the reference toner amount is made, and resetting a developing bias for realizing the reference toner amount based on the reconstructed relational characteristics and the reference toner amount. By this means, a high quality image can be obtained by suppressing color fluctuations with printing efficiency maintained at a high level even if large color fluctuations occur during continuous printing.