An internal combustion engine comprises a variable compression ratio mechanism capable of varying the mechanical compression ratio and a variable valve timing mechanism capable of controlling the closing timing of an intake valve. In an engine low-load operation, the mechanical compression ratio is maintained at a maximum mechanical compression ratio. In an engine high-load operation, the mechanical compression ratio is gradually decreased as the engine load increases. In the engine high-load operation, a load at which a predetermined mechanical compression ratio lower than the maximum mechanical compression ratio is obtained is preset, and a throttle valve is closed in the range of loads lower than the preset load.