An image forming apparatus prints images on print sheets, and insets tab-index sheets between the printed print sheets. The image forming apparatus includes an orientation detecting unit which detects an orientation of a print sheet being supplied, a rotation-angle selecting unit which selects a rotation angle of an image such that the image is arranged in an orientation matching the detected orientation of the print sheet and in a predetermined positional relationship with a tab of the tab-index sheets, and a printing unit which prints the image being rotated by the selected rotation angle.