PURPOSE: To prevent penetration of water and the like to a spark plug fixing hole by communicatively installing an atmospheric pressure adjusting chamber in an air vent communicating with a spark plug fixing hole to isolate the air vent from the outside air.

CONSTITUTION: A thin rubber bellows-shaped atmospheric pressure adjusting chamber 21 is communicatively installed in an air vent 20 to absorb the atmospheric pressure change in a spark plug fixing hole 5. A spark plug cap 10 stores air expanded within the fixing hole 5 by self heating in starting of an engine through the air vent 20. When the atmospheric pressure within the fixing hole 5 drops by cooling after stop of the engine, air stored in the adjusting chamber 21 returns to the fixing hole 5. The air vent 20 and the fixing hole 5 are completely isolated from the outside, and penetration of water and the like is prevented. Since the adjusting chamber 21 is integrally formed with a head cover 6 and the fixing hole 5 is sealed so as to communicate only with the air vent 20, even in washing with high pressure water, water penetration from the air vent 20 to the fixing hole 5 is prevented. A contact part 9 of the cover 6 is closely fit to a projecting wall 8, and a closely contact part 7 is brought into contact with the inner wall of the fixing hole 5 by pressure.