A grammage detection sensor according to an aspect of the present invention includes a transmission unit configured to transmit an ultrasonic wave, a reception unit configured to receive the ultrasonic wave transmitted from the transmission unit, and a control unit configured to detect a grammage of a recording material, when a plurality of recording materials are continuously conveyed between the transmission unit and the reception unit, on the basis of a first ultrasonic wave received by the reception unit after the ultrasonic wave is transmitted by the transmission unit between a preceding recording material and a recording material following the preceding recording material, and a second ultrasonic wave received via a recording material by the reception unit after the ultrasonic wave is transmitted by the transmission unit.