Patent ID: 8672722

Claim:
A method of manufacturing a spark plug comprising an insulator having an axial bore extending in a direction of an axis; a center electrode inserted into the axial bore; a metallic shell disposed externally of an outer circumference of the insulator and having a ledge projecting into interior thereof; a ground electrode joined to a forward end portion of the metallic shell and forming a gap in cooperation with the center electrode; and a packing disposed between the insulator and a surface of the ledge located on a rear side thereof opposite a forward side of the metallic shell where the gap is formed, the method comprising: a defect judgment step of judging whether or not the insulator has a defect, by means of generation of an electric potential difference between the center electrode and the metallic shell under conditions such that an assembly of the center electrode, the metallic shell, and the insulator is disposed within a pressure vessel, a high-pressure atmosphere higher in pressure than the atmospheric pressure is established within the pressure vessel, a space which allows the presence of insulating oil is a space surrounded by the packing, the metallic shell, the insulator, and an imaginary plane containing a forward end surface of the metallic shell, and the insulating oil is present at least in a region of the space where a distance between the ledge and the insulator becomes shortest.