PURPOSE: To prevent carbon from sinking when a spark plug is mounted on a cylinder head and to prevent disconnection, etc., of the plug during travel by providing a groove at one end of the externally threaded part of a spark-plug assistor, and providing an uneven part in the lower portion of the flange of the externally threaded part.

CONSTITUTION: In a spark-plug assistor 1 for use in attaching a spark plug 6 to a cylinder head 12, an axially extending groove 3 is formed at one end of an externally threaded part 2 screwed into the cylinder head 12, and an uneven portion 5 is formed in the lower portion 4 of the flange of the externally threaded part 2. A cylindrical member 8 is fitted around the corrugated part 7 of the spark plug 6, and a nut 10 and bolt 11 are attached to a terminal part 9. By thus providing the groove 3, carbon clinging to the threaded part of the cylinder head 12 is removed and collected into the groove 3. The carbon collected in the groove 3 is made to function as a sealant, whereby compression leakage can be prevented.