PURPOSE: To obtain a hydraulic torque converter by combining a variable rotary hydraulic pump and a rotary hydraulic motor.

CONSTITUTION: An eccentric shaft 2 is directly connected to an input shaft 1. A rotor 3 having an outer shape of hypotrochoid curve is installed on an eccentric part of the eccentric shaft for slidingly rotate the eccentric part. A housing 4 has an inner shape corresponding to an outer envelope drawn by the outer shape of the rotor. There are also provided driving side rotary hydraulic pumps 14, 15 each having a characteristic of a waste seal 9, a discharge-rate controlling changeover valve 20 provided on each discharge port, and a driven side rotary hydraulic motor 16 having the same structure as the rotary hydraulic pump.