A fuel injection control apparatus of an internal combustion engine is disclosed. This apparatus is provided with a crank angle detector that includes a signal rotor having a number of tooth portions and a toothless portion. The crank angle detector outputs a first signal corresponding to each tooth portion and a second signal corresponding to the toothless portion. A control computer selectively carries out a first calculation process for calculating the timing of fuel injection using the first signal and a second calculation process for calculating the timing of fuel injection using the second signal. When carrying out the second calculation process, the control computer calculates the timing of fuel injection using the length of time gained by dividing the length of time gained through detection of the toothless portion by the number of tooth portions which can be aligned in the toothless portion.