An ultrasonic sensor in the invention includes an ultrasonic transmitter, an ultrasonic receiver, and a detector. The ultrasonic transmitter transmits pulse-shaped ultrasonic waves to a thin plate to excite the thin plate. The ultrasonic receiver receives direct waves and reflected waves among the ultrasonic waves propagating in the thin plate excited by the pulse-shaped ultrasonic waves, the direct waves propagating only in the thin plate, and the reflected waves radiating outward, then reflected by the object, and returning to the thin plate. The detector detects the object present near the thin plate on the basis of a difference between a time at which the ultrasonic receiver receives the direct waves and a time at which the ultrasonic receiver receives the reflected waves.