PURPOSE: To obtain an optimum image output by setting an image forming condition by taking the reading characteristic of the image into account.

CONSTITUTION: After an operator judging that an abnormality exists in a gradation characteristic places a reference original in which a standard color patch is formed on an original platen, he instructs the start of a gradation control by an operation part 217. A CPU 214 in which the reading of the reference original is instructed from the operation part 217 reads the coordinate location and luminance of the color patch and prepares a table converting the luminance signal value obtained for every color component (RGB) into a standard value based on a corresponded coordinate location. This table information is preserved in a RAM 215, the information is set as the luminance correction coefficient of a correction part 1108 and the luminance signal to be read when the adjustment of the subsequent gradation characteristic is executed is corrected. Next, the operator places the printed recording medium on the original platen and instructs the reading. The CPU 214 reads a test pattern, prepares the conversion table of an LUT 25 based on the reading result, and this table information is preserved in the RAM 215 and is set to the LUT 25.