In a sheet overlap detection apparatus, an amplification factor control unit changes the amplification factor of a second amplifier which amplifies the output from a first amplifier which receives the output from an ultrasonic receiver. A first overlap detection monitor output decision unit decides the output from the second amplifier as a sheet overlap detection monitor output when the amplification factor of the second amplifier is adjusted, and the output value of the second amplifier has become almost equal to a first set value while one sheet is sandwiched between an ultrasonic transmitter and the ultrasonic receiver. A second overlap detection monitor output decision unit decides the output from the third amplifier as a sheet overlap detection monitor output when the amplification factor of the second amplifier is adjusted, and the output value of the second amplifier has not become almost equal to the first set value, but the output value of the third amplifier has become almost equal to a second set value while one sheet is sandwiched between the ultrasonic transmitter and the ultrasonic receiver. An overlap detection unit detects, based on the output from the second or third amplifier, overlap of sheets which pass between the ultrasonic transmitter and the ultrasonic receiver.