Patent ID: 7845637

Claim:
A double feed detecting device comprising: a first detecting unit having an ultrasonic transmitting unit and an ultrasonic receiving unit which are opposed to each other and disposed across a conveyance path of a sheet member, said first detecting unit detecting overlapping of the sheet members which pass along the conveyance path; a second detecting unit having a light emitting unit and a light receiving unit which are opposed to each other and disposed across the conveyance path, and detecting light which is irradiated from the light emitting unit and transmitted through the sheet member by the light receiving unit, and outputting a signal corresponding to a received light quantity; and a discriminating unit which discriminates the double feed when the first detecting unit detects the overlapping of the sheet members and an output signal of the second detecting unit is equal to or lower than a set double feed determination threshold value; wherein the first detecting unit detects an air spacer on the overlapping portion of the sheet members, and wherein the discriminating unit estimates a thickness of the sheet member with the air spacer which passes along the conveyance path based on a function of the first detecting unit, and changes a setting of the double feed determination threshold value.