In an internal combustion engine, a variable compression ratio mechanism (A) able to change a mechanical compression ratio and a variable valve timing mechanism (B) able to control the closing timing of an intake valve (7) are provided. The mechanical compression ratio is held at a maximum mechanical compression ratio at the engine low load operation side and gradually made to decrease as the engine load increases at the engine high load operation side. Combustion is performed by a stoichiometric air-fuel ratio at the engine high load operation side, and combustion is performed by a lean air-fuel ratio at the engine low load operation side where the mechanical compression ratio becomes the maximum mechanical compression ratio.