A cooling device for an internal combustion engine, comprising: a cooling-water flow path through which cooling water for cooling an internal combustion engine passes; a switching unit provided on an outlet side of the cooling-water flow path, and configured to branch the cooling-water flow path into either a heat-exchanging flow path having a heat exchanger for cooling the cooling water disposed therein or a bypass flow path without the heat exchanger, or both; a circulation pump that delivers, to the cooling-water flow path, cooling water passing through the heat-exchanging flow path and the bypass flow path; an inlet temperature detector that detects a temperature of the cooling water supplied to the heat exchanger; and an outlet temperature detector that detects a temperature of the cooling water discharged from the heat exchanger; characterized by further comprising: a controller that controls distribution, in the switching unit, of a volume of flow of the cooling water to the heat-exchanging flow path and the bypass flow path, wherein the controller includes a storage unit that stores a correspondence map indicating a correlation among: the amount of thermal deformation occurring in the heat exchanger, a temperature difference between a cooling water temperature detected by the inlet temperature detector and a cooling water temperature detected by the outlet temperature detector, and a volume of flow of the cooling water supplied to the heat-exchanging flow path, and when the cooling water is supplied to the heat-exchanging flow path, the controller refers to the temperature difference and the correspondence map to obtain the volume of flow of the cooling water supplied to the heat-exchanging flow path so that the amount of thermal deformation is less than or equal to a threshold amount of thermal deformation set in advance, and controls distribution, by the switching unit, of the volume of flow of the cooling water to achieve the obtained volume of flow of the cooling water. a cooling-water flow path through which cooling water for cooling an internal combustion engine passes; a switching unit provided on an outlet side of the cooling-water flow path, and configured to branch the cooling-water flow path into either a heat-exchanging flow path having a heat exchanger for cooling the cooling water disposed therein or a bypass flow path without the heat exchanger, or both; a circulation pump that delivers, to the cooling-water flow path, cooling water passing through the heat-exchanging flow path and the bypass flow path; an inlet temperature detector that detects a temperature of the cooling water supplied to the heat exchanger; and an outlet temperature detector that detects a temperature of the cooling water discharged from the heat exchanger; characterized by further comprising: a controller that controls distribution, in the switching unit, of a volume of flow of the cooling water to the heat-exchanging flow path and the bypass flow path, wherein the controller includes a storage unit that stores a correspondence map indicating a correlation among: the amount of thermal deformation occurring in the heat exchanger, a temperature difference between a cooling water temperature detected by the inlet temperature detector and a cooling water temperature detected by the outlet temperature detector, and a volume of flow of the cooling water supplied to the heat-exchanging flow path, and when the cooling water is supplied to the heat-exchanging flow path, the controller refers to the temperature difference and the correspondence map to obtain the volume of flow of the cooling water supplied to the heat-exchanging flow path so that the amount of thermal deformation is less than or equal to a threshold amount of thermal deformation set in advance, and controls distribution, by the switching unit, of the volume of flow of the cooling water to achieve the obtained volume of flow of the cooling water. a controller that controls distribution, in the switching unit, of a volume of flow of the cooling water to the heat-exchanging flow path and the bypass flow path, wherein the controller includes a storage unit that stores a correspondence map indicating a correlation among: the amount of thermal deformation occurring in the heat exchanger, a temperature difference between a cooling water temperature detected by the inlet temperature detector and a cooling water temperature detected by the outlet temperature detector, and a volume of flow of the cooling water supplied to the heat-exchanging flow path, and the amount of thermal deformation occurring in the heat exchanger, a temperature difference between a cooling water temperature detected by the inlet temperature detector and a cooling water temperature detected by the outlet temperature detector, and a volume of flow of the cooling water supplied to the heat-exchanging flow path, and when the cooling water is supplied to the heat-exchanging flow path, the controller refers to the temperature difference and the correspondence map to obtain the volume of flow of the cooling water supplied to the heat-exchanging flow path so that the amount of thermal deformation is less than or equal to a threshold amount of thermal deformation set in advance, and controls distribution, by the switching unit, of the volume of flow of the cooling water to achieve the obtained volume of flow of the cooling water.