PROBLEM TO BE SOLVED: To provide an internal combustion engine capable of being driven in uniform air/fuel ratio conditions, in a lean mixture and at good fuel consumption, even when a plug assembly and/or a cylinder head are in different combination, and a spark plug assembly suitable thereto. SOLUTION: A spark plug 10 includes a center electrode 17, a metal shell 11 having a mainbody 12 and a tip 15 smaller in diameter than the mainbody 14 at its end and an outside electrode 16 fixed to the end face 15a of the tip. The mainbody at its end is formed with a protrusion 14t fitted to a key way formed corresponding to the mainbody at its core end in a plug mounting hole at only one place when the spark plug is rotated around a shaft while being inserted in the plug mounting hole in a cylinder head. A relationship of a peripheral angle between the fixation portion 16a of the outside electrode and the protrusion is 180 degrees, almost the same as that of any spark plugs of the same type. The plug is assembled in the cylinder head formed where the key way corresponds to the stream line of a swirl.
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