A plug hole pipe 79 has thread grooves 81 formed on an outer circumferential surface thereof and causes a spark plug 11 to be pressed against a cylinder head 51 when the plug hole pipe is fixedly attached to the cylinder head 51 through screw engagement. The spark plug 11 is configured such that no thread grooves are formed on the outer circumferential surface of a front end portion 25 of a metallic shell 19, and can be vertically inserted into a plug disposition hole 51a formed in the cylinder head 51. By vertically inserting the spark plug 11 into the plug disposition hole 51a while the orientation of a ground electrode 21 is set beforehand in relation to the cylinder head 51, the ground electrode 21 can be oriented in a specific direction.