The object of the invention is to impart a suitable target slip amount depending on a state of change in an output from a driving power source to a torque converter, while simplifying control. A controller has a calculation part for calculating an engine torque change rate that is a rate of change in the output, and a target slip addition amount calculation part for calculating a negative value as an addition amount, when the engine torque change rate is less than or equal to a predetermined value that is a positive value, and for calculating a positive value as the addition amount, when the engine torque change rate is greater than the predetermined value. The controller is configured to set the target slip amount of a lockup clutch by arithmetic processing that integrates the addition amount, determined based on the calculated engine torque change rate, every control period.