Abstract: A water injection amount control system for a fuel and water injection engine, comprises running state detecting unit for detecting the running state of the engine; an EGR system for recirculating part of exhaust gas of the engine to a combustion chamber of the engine; EGR system operating state detecting unit for detecting or estimating the operating state of the EGR system; water injection amount regulating unit for regulating an amount of water to be injected to the combustion chamber of the engine; and control unit for controlling the operation of the water injection amount regulating unit: wherein the system is arranged to have water injection amount setting unit for deciding a water injection amount based on information from the running state detecting unit and on the operating state of the EGR system detected by the EGR system operating state detecting unit, so that the control unit controls the operation of the water injection amount regulating unit based on the water injection amount decided by the w