Gasoline direct injection (DI) has been widely utilized in high-compression-ratio engines and supercharged engines to improve the thermal efficiency. One of the most important quality problems in DI gasoline engines is the spark plug fouling due to dispersed fuel droplets under the fuel-rich condition during the cold start. To understand and solve this problem, we developed a new method to measure the amount of fuel droplets around the spark plug using a spark-plug-type optical sensor based on the light extinction. We confirmed that we could measure the amount of fuel droplets using this sensor. We found that the amount of fuel droplets strongly correlated with the number of cycles which the engine was operated until it could not restart at the pre-delivery test and the number of misfires during the warm-up. Therefore, the development of the spark plug antifouling performance is accomplished by controlling the amount of fuel droplets around the spark plug.