Spark plug and method for manufacturing the same

A spark plug, comprising:

• a tubular metal shell having an external thread formed on a part of an outer peripheral surface of the metal shell, and a tapered portion that projects radially outward with respect to the external thread and is provided on a rear end side of the metal shell with respect to the external thread;
• a center electrode held in an insulated manner at a center of the metal shell on a front end side thereof;
• and
• a ground electrode connected to the metal shell to form a spark gap between the ground electrode and the center electrode,
• wherein the tapered portion has a contact portion that contacts an inner surface of a plug hole formed in an engine when the external thread is tightened into a threaded hole in the plug hole, and
• wherein a Vickers hardness of the contact portion is higher than a Vickers hardness of at least a part of a portion between the contact portion and the external thread of the metal shell.