Additive information for restoring the contour of an image is generated and embedded in the image data. The format of the additive information is determined beforehand. When a hard copy is produced, the additive information is extracted from received image data. The additive information is for example vary small characteristic points of a density different from the density assigned for image data and arranged inside the contour. The image data is restored according to the extracted additive information. The additive information can be generated for example from the code information for generating patterns such as control points data of an outline font. For a half-tone image, a plurality of density bands is assigned to the additive information. A read error is detected by comparing the restored image with the received image data.