To provide an ultrasonic sensor in which an ultrasonic vibrator can be easily assembled having a circular vibration surface, a cylindrical cushion member, and a case.

When an ultrasonic component 55 is inserted, guiding convex portions 31, 33 slide inside guiding notches 41, 43. In this way, the ultrasonic component 55 moves along an axial direction without rotating. Then, if the cushion side lock 23 of a cushion member 17 abuts on the engaging convex portion 45 of a fitting hole 39, the portion 23 bends inside, and its claw 27 overcomes the portion 45 and returns due to its resiliency. Then the claw 27 engages with the portion 45. At the same time, in a rear end surface of the cushion member 17, a portion lack of the lock 23 abuts on the front end surface of the bulge 49 of a case 29. The ultrasonic component 55 can be prevented from moving in an axial direction.