When a regular reflection light receiving unit is receiving a reflected light in a non-image area where the test patches are not formed, a clamp switch is closed. Then, a first reference voltage generated by a pull-up resistor and a zener diode is corresponded to an electric potential of a terminal of a capacitor on the side of an A/D converter. The capacitor is charged by the difference in electric potential between the first reference voltage and an output voltage from the regular reflection light receiving unit. Next, the clamp switch is opened. After the regular reflection light receiving unit receives the reflected light from an image area where the test patches are formed with this state of things, the output voltage from the regular reflection light receiving unit is changed. The difference in density between the image area and the non-image area can be quantified by the A/D converter.