A practical detection system of engine load is required especially for a spark advance control system. We have successfully developed a system to estimate Indicated Mean Effective Pressure (hereafter, IMEP) by detecting the crankshaft angular velocity variation during one cycle of a four-stroke single-cylinder gasoline engine. This system has been commercially applied to the spark advance control system for small-displacement motorcycles. The system consists of the conventional hardware and the modified software. This paper clarifies the possibility of IMEP estimation using the crankshaft angular velocity variation based on the experiments and theoretical considerations.