An ultrasonic sensor includes an ultrasonic wave transmitting and receiving device, an elastic holding member elastically supporting the ultrasonic wave transmitting and receiving device, and a sensor case housing and holding the elastic holding member. A case cylindrical portion of the sensor case includes an engagement protrusion provided to protrude inward in a radial direction. The elastic holding member includes a support bottom portion in contact with a transmitting-and-receiving-device bottom, an engagement recess provided to be recessed inward in the radial direction and thus house the engagement protrusion, and a leading projection provided at a position corresponding to the engagement recess with respect to a peripheral direction surrounding a central axis line, and provided to protrude from the support bottom portion toward the base-end side in the axial direction. The support bottom portion is formed as a thick portion. The engagement recess is provided in the support bottom portion.