PURPOSE: To surely prevent the movement of a spark plug cylinder and prevent the lowering or the like of a seal performance by forming cam shaft housings so that when cam shafts are fixed at a cylinder head, the positioning of the spark plug cylinder is carried out by coming into contact with its flange portion.

CONSTITUTION: An engine 2 possesses a cylinder block, a cylinder head 4 which is furnished on its upper surface and defines a combustion chamber 8 on the inside, and a cylinder head cover 6 which is furnished at its upper part and forms cam chambers 10 on the inside. A spark plug cylinder 14 is put under pressure into a hole portion 12 formed in parallel with the vertical center line C of the cylinder head 4. In this instance, a flange portion 16 is formed at the outer periphery portion of the spark plug pipe 14. Cam shaft housings 20-1, 20-2 are formed so that when two cam shafts 18-1, 18-2 are fixed at the cylinder head 4, the positioning of the spark plug cylinder 14 is carried out by coming into contact with the flange portion 16. As a result, the longitudinal movement of the spark plug cylinder 14 is surely prevented.