An image processing apparatus including an exposing device, a developing device, a density measuring device to measure a density value of the developed film sheet, a calibrating device to calibrate the density value based on measured density values of a test image by the density measuring device and by an external density measuring device, a condition change detecting device to detect a condition change of the density measuring device, a correcting device to correct the density value, according to a result of detection by the condition change detecting device and the characteristics variation table showing variations of unmeasured values due to a condition change of the density measuring device, and a controlling device to control at least one of the exposing device and the developing device to optimize a relationship between diagnostic image data and film density according to the corrected density value.