The ultrasonic welding device includes a rotary support mechanical assembly rotatable about a rotation axis and operable to support a sheet over a circumference centered on the rotation axis, the sheet being continuously supplied； and a horn and an anvil operable to sandwich the sheet supported over the rotary support mechanical assembly to thereby weld the sheet. The rotary support mechanical assembly includes a rotary member rotatable about the rotation axis, a plurality of sheet support members each having a support surface for supporting the sheet, and a mounting section for allowing the plurality of sheet support members to be attached to the rotary member in a state that the sheet support members lie on the same circumference about the rotation axis, and allowing attaching positions of the sheet support members to the rotary member to be adjusted in radial directions centered on the rotation axis.
