In a cylinder head of an internal combustion engine, a plug hole for mounting an ignition plug is formed along a cylinder axis and an intake port and an exhaust port are respectively formed at both sides of the plug hole while sandwiching the cylinder axis therebetween. The cylinder head further includes an intake valve which opens or closes the intake port and an exhaust valve which opens or closes the exhaust port. In such a constitution, a cross-sectional shape of a flow passage of an outlet of the exhaust port is an oval shape extending in the horizontal direction and a water drain hole of the plug hole is formed below the exhaust port.