PURPOSE: To spread a degree of freedom in a control enabling the characteristic to be freely changed from the outside by providing a lockup clutch which directly connects a converter cover to an output shaft through a hydraulic clutch when the second pump impeller is connected.

CONSTITUTION: When a hydraulic control from the outside places in a released condition a hydraulic clutch 9 and a lockup clutch 19, as a pump impeller 2, only the first pump impeller 2a operates obtaining a small capacity coefficient depending on only this pump impeller 2a. While the hydraulic control from the outside places the hydraulic clutch 9 in a connected condition, when the lockup clutch 19 is placed in the released condition, as the pump impeller 2, the first and second pump impellers 2a, 2b operate obtaining a large capacity coefficient relying on both the pump impellers 2a, 2b. Further, when the hydraulic control from the outside places in the connected condition both the hydraulic clutch 9 and the lockup clutch 19, a lockup condition, directly connecting input and output shafts transmitting drive torque, is obtained by directly connecting a converter cover 1 to the output shaft 4 through both the clutches 9, 19.