An image process capable of rapidly detecting and correcting the inclination of a document image is executed as follows. Dots are put on a document image at even intervals and the document image is printed to provide a printed object. The printed object is read with a scanner to provide image data. Processes are performed on the image data with a plurality of filters for detecting dots corresponding to a plurality of inclinations to calculate the degrees of agreement between the data and the filters. The inclination angle of a filter exhibiting a highest degree of agreement is set to the inclination angle.