Factory adjustment and repairs are simplified while assuring the accuracy of a multifeed prevention function. A main unit has a conveyance path that conveys sheets, and a processing unit that applies a specific process to the sheets conveyed through the conveyance path, and a multifeed detection unit is removably disposed to the main unit. The multifeed detection unit has a sheet-passing unit disposed contiguously to the conveyance path, an ultrasonic transmitter disposed facing the sheet-passing unit, and an ultrasonic receiver disposed opposite the ultrasonic transmitter with the sheet-passing unit therebetween.