A cooling system for an internal combustion engine, comprising: a radiator configured to radiate heat from cooling water for the internal combustion engine; a radiator side cooling water route (2,11,12,13) configured to circulate the cooling water through the radiator and the internal combustion engine; a bypass side cooling water route (2,11,12,14) configured to circulate the cooling water through the internal combustion engine while detouring the radiator; a changeover device (15,21) configured to allow the cooling water to flow through the radiator side cooling water route (2,11,12,13) and the bypass side cooling water route (2,11,12,14) if a temperature of the cooling water for the internal combustion engine exceeds a threshold value and configured to allow the cooling water to flow through the bypass side cooling water route (2,11,12,14) while not allowing the cooling water to flow through the radiator side cooling water route (2,11,12,13) if the temperature of the cooling water for the internal combustion engine falls below the threshold value; a shutter configured to open/close on a flow passage for air when the air passes through the radiator; and a control unit configured to control the shutter and further configured to: increase an opening degree of the shutter in response to the load exerted on the internal combustion engine falling below a predetermined load; and reduce the opening degree of the shutter in response to the load exerted on the internal combustion engine exceeding a predetermined load if the cooling water in the radiator is below a prescribed temperature; wherein the prescribed temperature is greater than the threshold temperature. a radiator configured to radiate heat from cooling water for the internal combustion engine; a radiator side cooling water route (2,11,12,13) configured to circulate the cooling water through the radiator and the internal combustion engine; a bypass side cooling water route (2,11,12,14) configured to circulate the cooling water through the internal combustion engine while detouring the radiator; a changeover device (15,21) configured to allow the cooling water to flow through the radiator side cooling water route (2,11,12,13) and the bypass side cooling water route (2,11,12,14) if a temperature of the cooling water for the internal combustion engine exceeds a threshold value and configured to allow the cooling water to flow through the bypass side cooling water route (2,11,12,14) while not allowing the cooling water to flow through the radiator side cooling water route (2,11,12,13) if the temperature of the cooling water for the internal combustion engine falls below the threshold value; a shutter configured to open/close on a flow passage for air when the air passes through the radiator; and a control unit configured to control the shutter and further configured to: increase an opening degree of the shutter in response to the load exerted on the internal combustion engine falling below a predetermined load; and reduce the opening degree of the shutter in response to the load exerted on the internal combustion engine exceeding a predetermined load if the cooling water in the radiator is below a prescribed temperature; wherein the prescribed temperature is greater than the threshold temperature.