Disclosed is an internal combustion engine, which has a geometric compression ratio of 13.0 or greater, and a combustion chamber (4) configured to satisfy a condition of S / V2 0.12 (mm<-1>) when a radius r of a hypothetical sphere (IS) with its center at an ignition point (CP) of a spark plug (3) is set to satisfy a condition of V2 = 0.15 V1, where: S (mm<2>) is an area of an interference surface between the hypothetical sphere (IS) and an inner wall of the combustion chamber (4) in a state when a piston (30) is at its top dead center position; V1 (mm<3>) is a volume of the combustion chamber 4 in the state when the piston (30) is at the top dead center position; and V2 (mm<3>) is a volume of a non-interference part of the hypothetical sphere (IS) which is free of interference with the inner wall of the combustion chamber (4) when the piston (30) is at the top dead center position. The internal combustion engine of the present invention can more reliably improve fuel economy.