A method of image forming comprises the steps of: converting image data into coded data with a code string using a coding key; forming a small pattern string on a sheet of printing paper by printing small patterns, each small pattern corresponding to a code constructing the coded data; recording a coding key or data corresponding to the coding key on a recording medium combined with the printing paper; advising a user to input a decoding key; reading out recorded data from the recording medium; determining whether the decoding key corresponds with the recorded data; scanning the small pattern string recorded on the printing paper when the decoding key corresponds with the recoded data; converting the small pattern string scanned from the printing paper into the code string; decoding the code string into the original image data using the decoding key; and printing an image on a sheet of printing paper based on the image data.