An ultrasonic sensor includes a cylindrical case with a bottom. A piezoelectric element is bonded to an inner side of a bottom portion of the case, and a felt is bonded to the piezoelectric element. A cap is fitted to an opening of the case. The cap includes a fixing portion and a substrate which is harder than the fixing portion. Terminals are press-fitted to the substrate. Wires are connected to the terminals, so that the terminals are electrically connected to the piezoelectric element. A through hole is provided in the fixing portion. Resin before being foamed is injected from the through hole and then foamed, so that the case is filled with foamable resin while an excess amount of foamable resin is pushed out from the through hole.