Abstract: An EGR gas cooling system of the present invention includes a main cooling water circuit which circulates cooling water between an engine, a main radiator that radiates heat of the cooling water of the engine and a primary EGR cooler that cools EGR gas by the cooling water and a low temperature cooling water circuit which circulates the cooling water between the engine, a sub-radiator provided integrally with or separately from the main radiator to radiate heat in the cooling water and a secondary EGR cooler that cools the EGR gas by the cooling water. A flow rate controller which controls a flow rate of the cooling water is provided between the engine and the sub-radiator of the low temperature cooling water circuit.