A sheet detecting device including: an ultrasound generator; a transmitting side opening, which includes a first opening and a second opening to transmit the ultrasound; a transmitting side propagation unit, which is connected to the ultrasound generator and each of the transmitting side openings; a receiving side opening, which includes a third opening and a fourth opening to receives the ultrasound transmitted from the first opening and the second opening; an ultrasound receiver; a receiving side propagation unit, which is connected to each of the receiving side openings and the ultrasound receiver; and a first determining unit, wherein a first distance from the ultrasound generator to the first opening is different from a second distance from the ultrasound generator to the second opening, and wherein a third distance from the ultrasound receiver to the third opening is different from a fourth distance from the ultrasound receiver to the fourth opening.