An oil passage structure of a chain-driven oil pump includes three lines of oil passages for supplying oil from the center support to the torque converter, a first oil passage for supplying oil to the torque converter through the inside of an input shaft of the torque converter, a second oil passage for supplying oil to the torque converter through a space defined by the outer periphery surface of the input shaft and the inner periphery surface of a stator shaft, and a third oil passage for supplying oil to the torque converter by bypassing the drive sprocket. The third oil passage is provided on the outer periphery side of the drive sprocket and in a position where it does not interfere with the operation of a chain. Accordingly, it is possible to utilize the flex lock-up to a maximum extent, and to reduce the diameter of the sprocket.