A fuel injection device for Diesel engines includes a combustion detector comprising a photoelectric transducer and a light path extended through a nozzle body to transmit the light of a combustion flame developed in an engine cylinder to the photoelectric transducer. One end of the light path is opened to an oil sump communicating with a nozzle opening so that only when the nozzle opening is opened for injecting fuel and then a combustion flame is developed, the light of the flame is passed through the light path and reaches the photoelectric transducer thereby generating an electric signal indicative of the actual time of the combustion. The whole or part of the light path may be comprised of optical fibers.