An image forming apparatus includes an image forming device configured to form a first image on a first side of a recording medium and a second image on a second side of the recording medium, a position detector configured to detect respective positions of the first and second images, and a controller configured to perform, based on detection results obtained by the position detector, at least one of an image position correction by matching the first and second images and a magnification error correction by calculating and correcting a magnification error of one of the first and second images relative to the other. A program product used in the image forming apparatus includes a method of forming the first image and the second image, detecting the respective positions of the first and second images, and matching a position or a size of the first and second images.