To surely detect a double feeding for any kind of sheet materials in a low temperature and low humidity environment and in any other environmental conditions.

This sheet material double feeding detector comprises first and second transfer belts 7 and 9 applied across two rollers 8a and 8b and 10 a and 10b in loop shape and having a plurality of vent holes 24 and 25 formed therein, an ultrasonic wave transducer 17a for signal transmission in the loop of the first transfer belt for radiating ultrasonic wave to the sheet material transferred between the first and second transfer belts by the rotation of drive rollers 8a and 10a, an ultrasonic wave transducer 17b for signal receiving in the loop of the second transfer belt disposed at a position opposed to the ultrasonic wave transducer for signal transmission through a sheet material and receiving the ultrasonic wave transmitted through the sheet material, an upper duct 14 opening to the sheet material side of the first transfer belt, a lower duct 15 opening to the sheet material side of the second transfer belt, and an air sucking part 12 communicating with the upper and lower ducts.