A method is provided to manufacture a fuel injection control system controlling fuel injection in an internal combustion engine with a fuel injection valve. A fuel injection valve is trial-manufactured. Using this valve, correcting amounts for an injection period are calculated at plural adjustment points defined by fuel pressures and injection periods. The adjustment points are divided into actual-measuring points and predicting points, and the correcting amounts at the actual-measuring points are used to calculate predicting expressions to predict correcting amounts at the predicting points and relational expressions to define a relationship among the correcting amounts at the actual-measuring points. When the valve is mass-manufactured, for each mass-manufactured valve, the correcting amounts at the actual-measuring points and the predicting expressions are used to predict correcting amounts at the predicting points, providing the correcting amounts at the actual-measuring and correcting points. Appropriateness of the predicting expressions is also estimated.