1. An image forming apparatus comprising: an output image forming unit that forms an output image from an image-repeat image which is obtained by arranging a plurality of input images on one recording medium based on predetermined repeat conditions, the repeat conditions including a first vertex indicating a start point, a second vertex, a third vertex, and a fourth vertex indicating an end point of a rectangular area, the repeat conditions serving as magnification for a size of an original document; a preview image generating unit that generates reduced images of the input images, sets arrangement positions of the reduced images based on the repeat conditions, and arranges the reduced images in the arrangement positions in accordance with the repeat conditions, thereby generating a preview image that comprise a plurality of preview child images; a display unit that displays the preview image generated by the preview image generating unit; and a control unit that calculates coordinates of the second vertex and the third vertex of the rectangular area based on coordinates of the start point and coordinates of the end point of the rectangular area, the control unit calculates the coordinates of first vertexes, second vertexes, third vertexes, and fourth vertexes of preview child images based on the coordinates of the start point, the second vertex, the third vertex, and the end point of the rectangular area and based on the magnification designated as the repeat conditions, the control unit generates the preview child images, which are obtained by enlarging or reducing the rectangular area designated by the coordinates of the start point, the second vertex, the third vertex, and the end point of the rectangular area of the repeat conditions according to the magnification, the control unit calculates attachment positions of the preview child images based on the coordinates of first vertexes, second vertexes, third vertexes, and fourth vertexes of preview child images, the control unit attaches the preview child images to the attachment positions to generate the preview image for image-repeat printing, and the control unit thereby displaying the preview image on the display unit.