In determining the maximum injection timing most suitable for the time when the fuel injection reaches the maximum quantity at least as commensurate to an engine speed and an intake air pressure, the basic maximum injection timing most suitable for the time when the fuel injection reaches the maximum quantity under the atmospheric pressure as commensurate to an engine speed, and the basic maximum injection timing is corrected at least by an intake air pressure to provide the maximum injection timing, so that the maximum injection timing can be directly determined from an engine speed and an intake air pressure without determining the basic maximum injection quantity and the maximum injection quantity.