Double feed detecting apparatus for detecting double feed by ultrasonic, double feed detecting method, and recording medium having recorded therein program for executing double feed detection

A double feed detecting apparatus provided with an ultrasonic transmitter (2) installed on one side of a transport path for a sheet material (1) for transmitting an ultrasonic toward the sheet material (1), an ultrasonic receiver (3) installed on the other side of the transport path for the sheet material for receiving the ultrasonic transmitted by the ultrasonic transmitter and outputting a received ultrasonic signal, a first determining device for determining whether double feed or not in accordance with the amplitude of the received ultrasonic signal output by the ultrasonic receiver, and a second determining device for detecting a phase change in the received ultrasonic signal, and determining whether double feed or not on the basis of the detected phase change.