An image forming apparatus includes a sheet feeder, an image former, an ultrasonic sensor, and a controller. The image former forms an image on a sheet conveyed. The ultrasonic sensor is used to detect the sheet conveyed. The ultrasonic sensor includes a transmitter circuit which sends ultrasonic waves and a receiver circuit which receives the ultrasonic waves. The ultrasonic sensor outputs an output voltage in accordance with a strength of the ultrasonic waves received by the receiver circuit. The controller recognizes a current air pressure based on a magnitude of the output voltage.