Spark plug inspection method and spark plug manufacturing method

A spark plug inspection method for inspecting a spark plug including

• a cylindrical metal shell having an external thread formed on a part of an outer circumferential surface of the metal shell, and a tapered portion provided at a rear side of the metal shell with respect to the external thread and having a diameter that increases toward a rear side, the tapered portion being configured to come into contact with an inner surface of a plug hole formed in an engine when the external thread is tightened to an internal thread of the plug hole,
• a center electrode insulated and retained at a center on a front side of the metal shell, and
• a ground electrode connected to the metal shell and forming a spark gap between the ground electrode and the center electrode, the method comprising:
• a contact step of screwing the external thread to an internal thread of a gauge so as to bring a part of the tapered portion into line-contact or point-contact with an inner edge of the gauge;
• and
• a determination step of determining whether or not the ground electrode is present within a predetermined range in a circumferential direction of the gauge when the tapered portion comes into line-contact or point-contact with the inner edge in the contact step.