An accelerated state is detected as soon as possible at the engine start at which a crank pulse alone is insufficient to identify the stroke, and erroneous detection of the accelerated state is prevented. In a period from cranking start to stroke detection, data on suction air pressure is stored for each crank pulse in a virtual address, and during stroke detection, when the virtual address does not coincide with the normal address corresponding to the stroke, the data on the suction air pressure stored in the virtual address is transferred to the normal address, and thereafter the data on the suction air pressure is stored in the normal address, thereby making it possible to detect the accelerated state by making comparison, immediately after the stroke detection, with the suction air pressure prevailing one cycle before. Further, detection of an accelerated state is inhibited when the engine rpm variation is high wherein the suction air pressure increase state during the closure of the suction air valve does not become stable and also when the engine load is high.