A patch mark-forming unit forms a first patch mark at a first density on a surface. A light emitting unit emits an incident light onto the surface moving. The incident light reflected by the surface is divided into a mirror-reflected light and a diffusion-reflected light on the surface. A first detecting unit detects an amount of the diffusion-reflected light. The patch mark forming unit reforms a second patch mark at a second density weaker than the first density if the amount detected by the first detecting unit is larger than a threshold. A second detecting unit detects an amount of the mirror-reflected light reflected by the surface on which the second patch mark has been reformed. A position calculating unit calculates, based on the amount detected by the second detecting unit, a position at which an image should be formed. An image-forming unit forms an image at the position.