An internal combustion engine having a cylinder head body with a pentroof-shaped combustion chamber comprises a piston including a piston cavity and a caldera-shaped protrusion provided around the piston cavity, a fuel injector for injecting fuel downward to the piston cavity and a spark plug disposed obliquely in proximity to an intake valve. The piston cavity is provided on the top surface of the piston being offset on the spark plug side. Further, the caldera-shaped protrusion is slanted towards the spark plug side. Therefore, injected fuel collides against a down-slope of the piston cavity and is diffused partly towards the spark plug and partly towards the cylinder head. As a result, a locally rich air-fuel mixture is formed around the spark plug so as to enable stratified charge combustion.