An ultrasonic welding device for ultrasonically welding an object to be welded, comprising: a rotary support mechanical assembly (5a, 5c, 18, 44) rotatable about a predetermined rotation axis and operable to support the object over a circumference centered on the rotation axis, the object being continuously supplied; at least a pair of a horn and an anvil attached to the rotary support mechanical assembly (5a, 5c, 18, 44) to thereby revolve about the rotation axis and sandwich the object supported over the rotary support mechanical assembly (5a, 5c, 18, 44) to weld the object, when one of the horn and the anvil is defined as a first welding tool and the other of the horn and the anvil is defined as a second welding tool, the first welding tool being attached to the rotary support mechanical assembly (5a, 5c, 18, 44) in such a manner as to be movable relative to the second welding tool between a position to sandwich the object in cooperation with the second welding tool and a position away from the second welding tool; and a driving mechanism (3, 23) for driving the first welding tool in such a manner that the first welding tool moves relative to the second welding tool, characterized by a power transmission mechanism including a weight, and a power distribution mechanism (29, 30) for distributing a power generated by the driving mechanism (3, 23) in such a manner that the power is transmitted to the first welding tool and the weight in opposite directions, wherein the power distribution mechanism (29, 30) includes an endless belt, and a plurality of pulleys for bearing the endless belt in such a manner that two portions of the endless belt extend in parallel with each other, one of the two portions is connected to the driving mechanism (3, 23) and the weight and the other of the two portions is connected to the first welding tool, and the driving mechanism (3, 23) includes a cam drum having a circumferential surface formed with a cam groove, and a cam follower disposed in the cam groove and connected to the endless belt, the ultrasonic welding device further comprising a covering wall secured to the rotary support mechanical assembly (5a, 5c, 18, 44), and covering the circumferential surface of the cam drum and being rotatable relative to the cam drum, the covering wall being formed with a long hole for allowing the cam follower to move along the cam groove, wherein the power distribution mechanism (29, 30) includes a holding mechanism (26, 27, 31, 32, 33) attached to the covering wall in a state where the holding mechanism (26, 27, 31, 32, 33) covers the long hole, the holding mechanism (26, 27, 31, 32, 33) holding the endless belt and the plurality of pulleys. a rotary support mechanical assembly (5a, 5c, 18, 44) rotatable about a predetermined rotation axis and operable to support the object over a circumference centered on the rotation axis, the object being continuously supplied; at least a pair of a horn and an anvil attached to the rotary support mechanical assembly (5a, 5c, 18, 44) to thereby revolve about the rotation axis and sandwich the object supported over the rotary support mechanical assembly (5a, 5c, 18, 44) to weld the object, when one of the horn and the anvil is defined as a first welding tool and the other of the horn and the anvil is defined as a second welding tool, the first welding tool being attached to the rotary support mechanical assembly (5a, 5c, 18, 44) in such a manner as to be movable relative to the second welding tool between a position to sandwich the object in cooperation with the second welding tool and a position away from the second welding tool; and a driving mechanism (3, 23) for driving the first welding tool in such a manner that the first welding tool moves relative to the second welding tool, characterized by a power transmission mechanism including a weight, and a power distribution mechanism (29, 30) for distributing a power generated by the driving mechanism (3, 23) in such a manner that the power is transmitted to the first welding tool and the weight in opposite directions, wherein the power distribution mechanism (29, 30) includes an endless belt, and a plurality of pulleys for bearing the endless belt in such a manner that two portions of the endless belt extend in parallel with each other, one of the two portions is connected to the driving mechanism (3, 23) and the weight and the other of the two portions is connected to the first welding tool, and the driving mechanism (3, 23) includes a cam drum having a circumferential surface formed with a cam groove, and a cam follower disposed in the cam groove and connected to the endless belt, the ultrasonic welding device further comprising a covering wall secured to the rotary support mechanical assembly (5a, 5c, 18, 44), and covering the circumferential surface of the cam drum and being rotatable relative to the cam drum, the covering wall being formed with a long hole for allowing the cam follower to move along the cam groove, wherein the power distribution mechanism (29, 30) includes a holding mechanism (26, 27, 31, 32, 33) attached to the covering wall in a state where the holding mechanism (26, 27, 31, 32, 33) covers the long hole, the holding mechanism (26, 27, 31, 32, 33) holding the endless belt and the plurality of pulleys.