Abstract: A double feed detection method for detecting a double feed of a sheet-like detection object transported through a processing unit has an ultrasonic transmitter and an ultrasonic receiver positioned so as to sandwich a transport path along which the sheet-like detection object is transported so as to detect a double feed of the detection object. The double feed detection method has a step of forming a gap between the sheet-like detection objects where the sheet-like detection objects overlap when a double feed of the sheet-like detection objects occurs. Forming the gap between the sheets enhances an attenuation of an intensity of a response signal whenever a double feed occurs. As a result, a distinction between the intensity of the response signal when a double feed has occurred and when no double feed has occurred is sharpened and thus more easily detected, improving the double feed detection accuracy.