A control unit and a control method of an automatic transmission can accurately estimate an output shaft torque in a torque converter with little error. The control unit of the automatic transmission estimates an output shaft torque in a torque converter with using a characteristic of the torque converter and controls the automatic transmission by using the estimated torque converter output shaft torque. At lest one of a pump capacity characteristic and a torque ration characteristic of the torque converter is corrected in the control unit by using an oil temperature of the automatic transmission and at least one of parameters expressing an operating state, thereby estimating the torque converter output shaft torque.