Fuel injection quantity control system for engine

An injection quantity control system for an internal combustion engine (1) stores a value Qb, which is a sum of an FCCB correction value and an ISC correction value corresponding to an injection frequency of K times per combustion in multi-injection in an idling stabile state of the engine (1). The system stores a value Qc, which is a sum of the FCCB correction value and the ISC correction value corresponding to the injection frequency of N times. Based on a difference between the values Qb and Qc, a final learning value (Q1/K or Q1/N) is calculated. Thus, an injector individual difference, an inter-cylinder injection quantity variation, and an injector aging deterioration quantity can be discriminated from an increase in an engine-demanded injection quantity due to an engine load factor.