Spark plug and ignition device of internal combustion engine

An ignition device of an internal combustion engine comprising:

• a cylinder head of the internal combustion engine, the cylinder head with a plug hole formed in an inside of the cylinder head;
• a spark plug assembled with the plug hole of the cylinder head;
• and
• a press member pushing the spark plug toward the cylinder head, wherein the spark plug comprises:
• a spark plug housing having a cylindrical shape, an outer peripheral surface of which has no male screw part formed thereon;
• an insulator having a cylindrical shape inserted and fitted in the spark plug housing;
• a center electrode arranged in and supported by an inside of the insulator;
• and
• a ground electrode connected to the spark plug housing, a discharge gap being formed between the ground electrode and the center electrode, wherein:
• a housing support surface is formed on an inner peripheral surface of the spark plug housing, an outer peripheral surface of the insulator comprises an insulator support surface which is supported by the housing support surface in an axial direction of the spark plug;
• and
• the spark plug housing comprises:
• a large diameter part;
• a slope part having a pressure fitting part, the pressure fitting part having a taper shape in which a diameter of the pressure fitting part is reduced radially inwardly toward an inner peripheral side of the spark plug housing and from the large diameter part toward a front end part in the axial direction of the spark plug housing;
• and
• a small diameter part extending from the slope part toward the front end part in the axial direction of the spark plug housing, wherein:
• the pressure fitting part is pressed to a head pressure fitting part of the cylinder head of the internal combustion engine, a part of the pressure fitting part and a part of at least one of the housing support surface and the insulator support surface are arranged to be overlapped with each other in a radial direction of the spark plug;
• the press member pushes the spark plug to the cylinder head, the spark plug is supported between the press member and the head pressure fitting part, the pressure fitting part is configured to be pressed to the cylinder head of the internal combustion engine by pressure force in the axial direction of the spark plug housing of the press member;
• a length of the pressure fitting part is longer in the axial direction of the spark plug than a length of the insulator support surface of the insulator;
• and
• the large diameter part and the small diameter part are arranged to be spaced apart from the cylinder head.