The present invention relates to an ultrasonic sensor for detecting double sheets and a method of detecting double sheets using the same, in which in configuring the ultrasonic sensor for detecting double sheets of banknotes passing through a transfer path, the ultrasonic sensor is configured to include an ultrasonic wave generator and an ultrasonic wave receiver provided in pair to face each other in a direction perpendicular to a transfer direction of the banknotes with the transfer path for transferring the banknotes interposed therebetween, and periodicity of a reception signal detected by the ultrasonic wave receiver is determined to detect whether a zero sheet, a single sheet or double sheets of the banknotes pass through the transfer path according to generation or not of the periodic signal and/or a frequency of the generation.