According to the present invention, a monitoring system for an internal combustion engine includes a fuel injection control means for outputting a fuel injection signal to an injector based on a target fuel injection quantity and a target fuel injection timing according to a driving condition of the engine. The monitoring system further includes a computation means for computing an actual fuel injection quantity and an actual fuel injection timing based on the fuel injection signal, and a determination means for determining whether a fuel injection malfunction exists by comparing the actual fuel injection quantity and the target fuel injection quantity and comparing the actual fuel injection timing and the target fuel injection timing.