A cooling system of an internal combustion engine has a plurality of coolant passages located in a row along a longitudinal axial direction of a cylinder liner. Each of the plurality of coolant passages is located so as to extend approximately along a circumference of the cylinder liner. The plurality of coolant passages are located between an inner wall of a cylinder block and an outer wall of the cylinder liner. The cylinder liner is fitted in the cylinder block. The cooling system also has an inflow passage through which coolant flows into the plurality of coolant passages. The cooling system also contains a flux passage through which coolant flows out of the plurality of coolant passages. The inflow passage and the flux passage are arranged such that, the closer a coolant passage is to a combustion chamber end of the engine cylinder, the smaller the pressure loss of coolant flowing through a part of the inflow passage, that coolant passage and a part of the flux passage, the coolant traveling along that part of the inflow passage, that coolant passage and that part of the influx passage.