To provide a proper cooling method and system capable of solving a problem of need for cooling in a case where an electronic control device for shifting is provided in a housing of an automatic transmission, which has a deterioration/damage due to heat generation in semiconductors in the electronic control device caused by heat generation of transmission oil.

Oil temperature in the automatic transmission or electric part temperature in the electronic control device is detected, and a detected temperature is compared with a predetermined temperature. When the detected temperature is higher than the predetermined temperature, oil temperature inside a case of the automatic transmission is lowered by putting an electric fan on, forcedly connecting a lockup clutch, reducing pressure control value of a pressure regulator, or generating a gear shift ratio reducing warning or the like.