PURPOSE: To improve power performance during selection of a power mode, by a method wherein, during selection of a power mode detected by an operation mode detecting means, control of the fastening force of the lockup means of a torque converter is prohibited.

CONSTITUTION: The fastening force of a lockup means 5 of a torque converter 2 is controlled to a target value by a control means 20, and an engine power is excellently transmitted to a drive shaft through the lockup means 5 as transmission of the fluctuation in torque of an engine is effectively suppressed. In this case, when it is detected by an operation mode detecting means 21 that an operation mode is selected as a power mode, during demand of power performance, control of the fastening force of the lockup means 5 by means of the control means 20 is forcibly prohibited by a prohibiting means 22, the lockup means 5 is completely released, and the loss of an engine power is relieved by an amount equivalent to that of the lockup means 5. This constitution enables improvement of power performance during selection of power mode as vibration of a car body during normal running is reduced and fuel consumption is improved.