1. A control system for an internal combustion engine comprising: a compression ratio control mechanism capable of changing an engine compression ratio; and a control unit for controlling the compression ratio control mechanism in accordance with an acceleration state of the engine; the control unit including: an acceleration parameter obtaining section for obtaining an acceleration parameter corresponding to a change of an engine speed; an acceleration determining section for determining whether the engine is in a slow or fast acceleration state on the basis of the acceleration parameter; and a control section for controlling at least one of a compression ratio change speed at which the engine compression ratio during engine acceleration is changed and a compression ratio change start time at which a change of the engine compression ratio during engine acceleration is started, on the basis of whether the engine is in a slow or fast acceleration state.