An ultrasonic sensor includes a casing body and a piezoelectric element. The casing body has a base wall and a surrounding wall that cooperates with the base wall to define an interior space. The base wall has a top surface and a bottom surface. The top surface has a first section, a second section that extends between the first section and the surrounding wall, and a connecting section interconnecting the first and second sections. A distance between the second section and the bottom surface is larger than that between the first section and the bottom surface such that the connecting section and the first section cooperate to define a recess. The piezoelectric element is confined in the recess.