A control method of controlling the cooling water temperature of an engine mounted in a vehicle is provided in which the lower limit value of the engine rotation speed is set according to the outside air temperature and the vehicle speed. When the vehicle starts traveling, a temperature raising-state lower limit rotation speed is set to raise the cooling water temperature to a third threshold value (70°C). After the cooling water temperature is raised to the third threshold value, the lower limit value is not set. If the cooling water temperature turns to drop and drops to a second threshold value (65°C), a maintaining-state lower limit rotation speed lower than the temperature raising-state lower limit rotation speed is set to raise the cooling water temperature. Here, if the cooling water temperature does not rise but continues dropping and drops to a first threshold value (60°C), a temperature re-raising-state lower limit rotation speed lower than the temperature raising-state lower limit rotation speed and higher than the maintaining-state lower limit rotation speed is set.