Abstract: An ultrasonic wave that is transmitted from an ultrasonic-wave transmitter 1 is received by an ultrasonic-wave receiver 2, through a sheet 4. A reception signal is input to an extracting unit 19. The extracting unit compares its phase with a phase of a reference signal output from a reference-signal reproduction unit 18. It converts a phase difference therebetween into a signal having a level corresponding to the phase difference. The signal having a level corresponding to the phase difference is compared with a threshold value set in a threshold-value setter 20, by a comparator 21. When one sheet 4 is fed, the comparator 21 produces a signal of high level. When a doubles feeding occurs, it produces a signal of low level.