[PROBLEMS] To provide an ultrasonic sensor having an enhanced obstacle shortest detection distance resolution by attenuating the unwanted vibration of the sides of the case and reducing the reverberating time. [MEANS FOR SOLVING PROBLEMS] A piezoelectric element (3) is fixed to the inside surface of the bottom (1a) of a bottomed cylindrical outer case (1). An annular inner case (2) having a center hole (2b) extending in the axial direction inside the side wall of the outer case (1). A flat part (2d) is provided around the center hole (2b) of the inner case (2). A viscoelastic filler (6) is placed in the center hole (2b) to the flat part(2d). Plate members (7) are opposed on the flat part (2d), with the filler (6) interposed therebetween. When the inner case (2) vibrates, the plate members (7) hardly vibrate and therefore the filler (6) sandwiched between the flat part (2d) and the plate members (7) shear-deform, and the vibration is suppressed by the viscoelastic attenuation action.
