An ultrasonic sensor is disclosed. The ultrasonic sensor includes a piezoelectric element and an acoustic matching member. The piezoelectric element is configured to detect ultrasonic wave transmitted from a transmitter and reflected by a detection target object located in a detection target space. The acoustic matching member is configured to conduct the received ultrasonic wave to the piezoelectric element. The piezo electric element is covered with the acoustic matching member including a principal oscillation portion and a supplement oscillation portion. Thickness of a part of the supplement oscillation portion, the part covering the piezoelectric element, is smaller than a predetermined thickness threshold.