A controller (5) controls the engaging force between a pump impeller (1a)
connected to an engine (2) and a turbine runner (1b) connected to an
automatic transmission (23). The controller (5) first performs
feedforward control of the engaging force. The controller (5) determines
a reference value and a target rotation speed based on the capacity
characteristics of the torque converter (1), and if the rotation speed of
the engine (2) becomes less than the reference value, performs feedback
control of the engaging force so that the deviation between the target
rotation speed and the rotation speed decreases. When a predetermined
condition is satisfied, the controller (5) changes over from feedforward
control to feedback control, even if the rotation speed of the engine
(21) is not less than the reference value. As a result, the control
precision of feedback control when performing a slip lock-up, is
increased.