To provide a cooling system of a compression self-ignition type internal combustion engine for performing combustion by compression self-ignition in a stable state by controlling a wall surface temperature of a combustion chamber with excellent responsiveness with a relatively simple constitution.

This cooling system has a cooling water passage 9 for supplying cooling water to a cylinder block 2 and a cylinder head 3 for forming the combustion chamber 4 of the internal combustion engine 1, a water pump 10 for circulating the cooling water in the cooling water passage 9 and a radiator 11 for cooling the cooling water. The cooling water passage 9 has a radiator passage 12 connected to the upstream side of the water pump 10 and provided with a radiator 11 and a bypass passage 13 by bypassing the radiator 11, and also has opening-closing valves 14 and 15 for respectively opening-closing the radiator passage 12 and the bypass passage 13, a control means 8 for controlling operation of the opening-closing valves 14 and 15 and a storage tank 16 for storing the cooling water.