To realize miniaturization of a transformer for AC-DC conversion of DC power transmission as a device, and to provide a transformer for AC-DC conversion in which a harmonic current of 5, 7,... multiple harmonic waves will not flow into a tap changer.

In a transformer 3 having AC-side winding connected by star connection as well as DC-side winding are connected by star connection and having an on-load tap changer 35 provided therein, and a transformer 4 having AC-side windings connected by star connection and DC-side windings connected by delta connection and having an on-load tap changer 45 provided therein, tap connection leads 34U-34W and 44U-44W for connecting tap selectors 35sU-35aW and 45aU-45sW of the on-load tap changers 35 and 45 which are connected with respective taps of tap windings 33U-33W and 43U-43W, provided on neutral side of the AC-side windings 31U-31W and 41U-41W are connected in parallel between both transformer 3 and 4. Then currents flowing through the on-load tap changers 35 and 45 are made uniform so as to restrain the peak value of the current to a small value.