PURPOSE: To enable one light projecting/receiving device to handle all inspections of color missing, mismatching, deviation between two sheets and the like by detecting the amount of light transmission and the output voltage at the light detecting section when the paper fed is crossing the device.

CONSTITUTION: Crossing through the sheet paper 16, the light permeated is received by a light receiving section 28 and the signal from the section 28 enters an initial amplification circuit 38 and an analogue memory 40 to be memorized. It enters two volume controls via an amplification circuit 42 when a timing pulse is received. When the inspection signal of the intended sheet paper reaches comparators 34 and 36, the signal memorized in the analogue memory 40 also enters the comparators 34 and 36 and the reference signal via the volume control. The comparators are provided subsequently with an and circuit 44 and a not-or circuit 46 and the signal outputted from the comparators are checked up with the timing signal through the and circuits.