An ultrasonic sensor includes an ultrasonic element (22) and an element accommodation case (21). A side wall portion (211) of the element accommodation case (21) has a tubular shape surrounding a directional central axis. A bottom wall portion (212) and a top wall portion (213) of the element accommodation case (21) cover ends of the side wall portion in a direction along the directional central axis. The top wall portion (213) has a diaphragm portion (214). The bottom wall portion (212) supports the ultrasonic element (22). For example, the ultrasonic element (22) is opposed to the diaphragmportion (214) across a gap (G) defining an interval corresponding to an integral multiple of half of a wavelength of ultrasonic vibration. Alternatively, the diaphragm portion (214) is formed of a material having an acoustic impedance of 50*105 Pa*s/m or more and 5000*105 Pa*s/m or less, and has a thickness of 1 mm or less.