First and second light amount detection elements are arranged on a scanning line of a laser beam. The first light amount detection element receives a laser beam having a first light amount, and the second light amount detection element receives a laser beam having a second light amount that is lower than the first light amount. The detection signal that is output by the second light amount detection element is amplified by an amplifier and used as a synchronization signal. Here, an amplification factor of this amplifier is greater than an amplification factor of another amplifier for amplifying the detection signal that is output by the first light amount detection element. The detection signals amplified by the amplifiers are modified by a modification coefficient that correspond to the amplification factors, and used as signals that indicate the light amounts.