Common rail type fuel injection system

The system has a fuel supply pump (4) with a feed pump (11) for sucking fuel from a fuel tank. A common rail (2) stores the fuel pressurized by the pump. A relief passage returns the fuel in the rail to the tank. A relief valve (6) selectively opens and closes the passage. The valve is opened when rotational speed of an internal combustion engine is lower than a preset speed or an engine load is lower than a preset load. The pressurized fuel stored in the common rail is injected by a fuel injector (3) into a cylinder of the internal combustion engine.