An intake port and an exhaust port are formed without changing a passage sectional area or its shape a little and without increasing an entire height of an engine. A seat machining part of a valve guide fixing seat of the intake port and the exhaust port are arranged at the cylinder head and are bent in any one of the right side direction or the left side direction as seen from the center of the valve. With such an arrangement, a minimum thickness for the seat machining can be assured, the intake port and the exhaust port do not change the passage sectional area or the sectional shape a little and so it is not necessary to increase the entire height of the engine.