A controller (5) performs open loop control of the engaging state of a lockup clutch (2) through a switching mechanism (3, 4) when a torque converter (1) transitions from a first state in which the lockup clutch (2) is disengaged to a second state in which the lockup clutch (2) is at least partially engaged. At this time, the controller (5) estimates the engine torque at the time when open loop control ends, estimates a necessary lockup capacity required for the converter (1) at the time when open loop control ends, based on the estimated engine torque, and controls the engaging state of the lockup clutch (2) through the switching mechanism (3, 4) to make the lockup capacity at the time when open loop control ends become the necessary lockup capacity.