Spark plug and manufacturing method thereof

A spark plug, comprising:

• a center electrode extending in an axial direction;
• an insulator having an axial bore extending in the axial direction and holding the center electrode in the axial bore at a front end side;
• a cylindrical metal shell radially surrounding the insulator so as to hold the insulator and having a front end constituent face at a front end side opening thereof in which the front end constituent face, comprised of a visible external surface when viewed from a front end side in the axial direction, has a plurality of faces;
• and
• a ground electrode having one end joined to at least one of the plurality of faces that constitute the front end constituent face and other end bent toward an inner circumferential of the metal shell so as to form a spark discharge gap with a front end portion of the center electrode, the ground electrode further having an extending portion extending in the axial direction from the one end to the other end and a bending portion between the extending portion and the other end,
• wherein a virtual sphere is neither in contact with the center electrode nor the insulator, where the virtual sphere having a radius of 1.2 mm is assumed to be in contact with an inner face, which faces toward the center electrode, of the bending portion of the ground electrode,
• wherein, in the plurality of faces constituting the front end constituent face of the metal shell, a face adjoining at least a part of an inner circumferential face of the metal shell in the axial direction and constituting an inclined face where a diameter thereof increases from a rear end side to the front end side in the axial direction, serves as a first face, and
• wherein, on a cross-sectional outline of the metal shell including the axis thereof, a length of the first face is the longest in that of the plurality of faces, which constitute the front end constituent face.