To provide an image forming apparatus which carries out perfect image formation by correcting detected values even when wrong detection of right/left end positions near a front end and near a rear end of an image forming medium takes place, and to provide a method for correcting detected positions of an image forming medium.

Medium right/left end position detection data stored sequentially in a memory is used. Since an unstable region correction starting indication before an image formation starting timing of the medium front end is front end correction (YES in S71), a "front end unstable region" showing an unstable value is selected from the right/left end position detection data near the front end (S72), and a "front end stable region" showing a stable value after that is selected (S73). Detection data of the front end stable region is substituted for detection data of the front end unstable region. Moreover, since the unstable region correction starting indication before an image formation ending timing of the medium rear end is rear end correction (NO in S71), a "rear end unstable region" showing an unstable value is selected from the right/left end position detection data near the rear end (S75), and a "rear end stable region" showing a stable value before that is selected (S76). Detection data of the rear end stable region is substituted for detection data of the rear end unstable region.

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