Misfire detection system for an internal combustion engine. If misfire is found to occur due to vapor lock temporarily happening, it will erroneously be judged that the structure of the engine or the fuel supply system is out of order. However, vapor lock is an interruption of the flow of fuel in a gasoline engine caused by formation of vapor or gas bubbles, and does not mean the engine or the fuel supply system is degraded in its structure. On the contrary, if misfire is detected in such situation, the misfire detection degrades the reliability of the system. The misfire detection is therefore inactivated during conditions in which vapor lock is liable to occur, by discontinuing the detection, or by deliberately deeming no misfire to occurs even if it is so detected. Alternatively, the reference level for misfire detection is raised during such conditions.