Patent ID: 8313096

Claim:
An image reading apparatus comprising: an ultrasonic sensor; and a control unit including a receiving unit that receives an output of the ultrasonic sensor, a holding unit that holds the output received by the receiving unit as any one of an output when there is no paper, an output when there is no overlap of papers, and an output when there is an overlap of papers, a changing unit that changes a previously set threshold of multifeed determination to an optimal value for a thickness of the paper based on the output held as the output when there is no paper and the output held as the output when there is no overlap of papers by the holding unit, and a determining unit, when there is an output held as the output when there is an overlap of papers by the holding unit, compares the output and a threshold after it is changed by the changing unit, to perform multifeed determination, wherein the receiving unit, when a difference between an output received this time and an output received last time is equal to or greater than a predetermined value, sets the output received this time as a replacement candidate, and if the output received last time has been set as the replacement candidate and when the output received this time is closer to an output received before the last time than the output received last time, replaces the output received last time as the replacement candidate with an average value between the output received this time and the output received before the last time.