PURPOSE: To prevent the malfunction of T/M by selecting lock-up dot data for high oil temperature shifted to a slow-speed side to usual lock-up dot data when an oil temperature within a transmission detected with an oil temperature sensor exceeds a set temperature.

CONSTITUTION: An oil temperature within a transmission is detected with an oil temperature sensor 3 in an electronic control device 10, and when the oil temperature exceeds a set temperature previously set, lock-up dot data for a high oil temperature, shifted to a slow-speed side to usual lock-up dot data, is selected to expand a lock-up region. And lock-up control is made by outputting a lock-up signal to a lock-up solenoid 8 based on the lock-up dot data for a high oil temperature. Thus a T/M oil temperature can be lowered, and the malfunction of T/M having a torque converter can be prevented.