An ultrasonic sensor includes a plurality of vibrating parts, a plurality of receiving elements, and a waveguide. Each of the vibrating parts vibrates when a corresponding ultrasonic wave reflected by a detection object is transmitted thereto, and receives the ultrasonic wave. Each of the elements includes corresponding one of the vibrating parts and detects the object using the corresponding ultrasonic wave. The ultrasonic wave is transmitted through the waveguide to each of the elements. The waveguide includes a first opening facing the object, a second opening, and a reflecting surface that reflects the ultrasonic wave in a direction to each of the vibrating parts. The ultrasonic wave enters through the first opening. The second opening is not viewable from the first opening. The second opening holds the elements such that the each of the vibrating parts faces a direction where the each of the vibrating parts receives the ultrasonic wave.