To certainly prevent misalignment in print start timing, resulting from malfunction due to noise.

An LSU 6 includes three phototransistors 24A, 24B, 24C and a sensor output circuit 23. The phototransistors detect the quantity of light of respective light beams, and are arranged in the scanning direction on the outside of the printing area of a photoreceptor on the scanning start side. A sensor processing circuit 23 creates a pulse signal of fixed width by comparing the output signals from the phototransistors with each other. An image processing part 4 controls the LSU 6 by using the pulse signal as a print start signal when the width of the pulse signal falls within a predetermined range, otherwise by not using the pulse signal as a print start signal.