An electronic control unit (ECU) of an injection control system of an internal combustion engine measures an engine rotation speed in a period from a time point when an exhaust valve opens to a time point when a top dead center of a next cylinder is detected after a single injection is performed. The ECU calculates a rotation speed fluctuation caused by the single injection based on the engine rotation speed. The engine rotation speed provided immediately after the single injection is measured after a cylinder pressure increased by the single injection decreases to substantially the same level as the cylinder pressure provided in the case where the single injection is not performed. Therefore, the rotation speed fluctuation corresponding to torque generated by the single injection can be measured accurately.