To provide a fuel injection system with a rationally improved assembling property.

This common rail type fuel injection system is provided with a high-pressure fuel supplying pump 1 increasing a pressure of fuel and feeding it and a common rail 52 storing the fuel fed from the high-pressure fuel supplying pump 1 and injects the high-pressure fuel stored in the common rail 52 to a combustion chamber of an internal combustion chamber. An air compressor 20 pressuring air at high pressure and feeding it to a predetermined engine is integrally assembled to the high-pressure fuel supplying pump 1. The high- pressure fuel supplying pump 1 and the air compressor 20 are driven by means of a shaft rotating by driving force of the internal combustion engine.