An internal combustion engine comprising: a cylinder; a piston which reciprocates within said cylinder; a cylinder head which defines a combustion chamber in cooperation with said cylinder and piston; a spark plug which is provided in said cylinder head; and an intake valve and an exhaust valve each having a valve head (1a, 2a) which closely contacts with a corresponding valve seat (11a or 12a) of said cylinder head to seal said combustion chamber, wherein: said internal combustion engine has a valve overlap period for allowing both said intake and exhaust valves (1, 2) to be opened during the course of a cylinder cycle, and a geometric compression ratio of 13.0 or greater, said internal combustion engine being designed to satisfy, at a center timing (Tc) of said valve overlap period, a conditional expression: S1 ≥ S2, where: S1 is a cross-sectional area of said combustion chamber taken along any selected one of a plurality of mutually parallel hypothetical cutting-planes each of which extends parallel to a linear reciprocating direction (d1 or d2) of at least one of said intake and exhaust valves (1, 2) and passes through the valve head (1a or 2a) of said at least one of said valves (1, 2); and S2 is an effective opening area defined between the valve head (1a or 2a) of said at least one of said valves (1, 2) and the corresponding valve seat (11a or 12a) in a region on an outward side of said combustion chamber relative to said selected hypothetical cutting-plane. a cylinder; a piston which reciprocates within said cylinder; a cylinder head which defines a combustion chamber in cooperation with said cylinder and piston; a spark plug which is provided in said cylinder head; and an intake valve and an exhaust valve each having a valve head (1a, 2a) which closely contacts with a corresponding valve seat (11a or 12a) of said cylinder head to seal said combustion chamber, wherein: said internal combustion engine has a valve overlap period for allowing both said intake and exhaust valves (1, 2) to be opened during the course of a cylinder cycle, and a geometric compression ratio of 13.0 or greater, said internal combustion engine being designed to satisfy, at a center timing (Tc) of said valve overlap period, a conditional expression: S1 ≥ S2, where: S1 is a cross-sectional area of said combustion chamber taken along any selected one of a plurality of mutually parallel hypothetical cutting-planes each of which extends parallel to a linear reciprocating direction (d1 or d2) of at least one of said intake and exhaust valves (1, 2) and passes through the valve head (1a or 2a) of said at least one of said valves (1, 2); and S2 is an effective opening area defined between the valve head (1a or 2a) of said at least one of said valves (1, 2) and the corresponding valve seat (11a or 12a) in a region on an outward side of said combustion chamber relative to said selected hypothetical cutting-plane. said internal combustion engine has a valve overlap period for allowing both said intake and exhaust valves (1, 2) to be opened during the course of a cylinder cycle, and a geometric compression ratio of 13.0 or greater, said internal combustion engine being designed to satisfy, at a center timing (Tc) of said valve overlap period, a conditional expression: S1 ≥ S2, where: S1 is a cross-sectional area of said combustion chamber taken along any selected one of a plurality of mutually parallel hypothetical cutting-planes each of which extends parallel to a linear reciprocating direction (d1 or d2) of at least one of said intake and exhaust valves (1, 2) and passes through the valve head (1a or 2a) of said at least one of said valves (1, 2); and S2 is an effective opening area defined between the valve head (1a or 2a) of said at least one of said valves (1, 2) and the corresponding valve seat (11a or 12a) in a region on an outward side of said combustion chamber relative to said selected hypothetical cutting-plane. S1 is a cross-sectional area of said combustion chamber taken along any selected one of a plurality of mutually parallel hypothetical cutting-planes each of which extends parallel to a linear reciprocating direction (d1 or d2) of at least one of said intake and exhaust valves (1, 2) and passes through the valve head (1a or 2a) of said at least one of said valves (1, 2); and S2 is an effective opening area defined between the valve head (1a or 2a) of said at least one of said valves (1, 2) and the corresponding valve seat (11a or 12a) in a region on an outward side of said combustion chamber relative to said selected hypothetical cutting-plane.