an engine body having a geometrical compression ratio set at 13.5 or more, said engine body being operated using a fuel having a research octane number (RON) of 91 or more;

an intake valve and an exhaust valve which are provided, respectively, in intake and exhaust ports connected to each of a plurality of cylinders of said engine body, said intake and exhaust valves being adapted to open and close corresponding ones of said intake and exhaust ports;

operation-state detection means adapted to detect an operation state of said engine body; and

control means adapted, based on detection of said operation-state detection means, to perform at least an adjustment control of an ignition timing of said spark plug, and an adjustment control of an effective compression ratio by means of an adjustment control of a closing timing of said intake valve, said control means being operable, when the engine operation zone is a high load operation zone including a wide open throttle region, within at least a low speed range, to adjust a closing timing of said intake-valve in such a manner as to maintain the effective compression ratio at 12.5 or more, and retard said ignition timing to a point within a predetermined stroke range just after a top dead center of a compression stroke, wherein said effective compression ratio is calculated based on an intake-valve closing timing defined by a valve lift amount of 1 mm.