An ultrasonic sensor includes: an ultrasonic element provided to transmit or receive an ultrasonic wave propagating along a directional axis; and an element housing case that includes a case diaphragm having a thickness direction along the directional axis. A resonant space is defined between the case diaphragm and the ultrasonic element for the propagating wave, by housing the ultrasonic element while separating the ultrasonic element from the case diaphragm. A horn shape is defined in the element housing case in which a width of the resonant space in a direction orthogonal to the directional axis is reduced as the resonant space extends in an axial direction parallel to the directional axis.