An ultrasonic welding device for a secondary battery is provided. The ultrasonic welding device is configured to weld an overlapping surface of an electrode tab and an electrode lead in an electrode assembly. The ultrasonic welding device includes an anvil on which the overlapping surface of the electrode tab and the electrode lead are disposed. A horn is configured to apply ultrasonic waves to the overlapping surface of the electrode tab and the electrode lead while pressing the overlapping surface. The overlapping surface is disposed on the anvil. An electrode tab measuring part is configured to measure a thickness of the electrode tab disposed on the overlapping surface. A pressure adjusting device is configured to adjust a pressure applied to the overlapping surface by the anvil and the horn according to the thickness value of the electrode tab. The thickness value is measured by the electrode tab measuring part.