A welding system with an ultrasonic welder for welding a plurality of webs together while carrying the webs, wherein (a) the ultrasonic welder comprises: an anvil roller including a pair of anvils (11, 12); a first ultrasonic horn and a second ultrasonic horn that apply vibration energy to the webs in cooperation with the pair of anvils; and carrying means for carrying the webs so that the webs pass a first gap between the anvil roller and the first ultrasonic horn and then pass a second gap between the anvil roller and the second ultrasonic horn, wherein the first and second ultrasonic horns are arranged so that one anvil of the pair can face the first ultrasonic horn at the same time as the other anvil of the pair faces the second ultrasonic horn, wherein the first and second ultrasonic horns apply the vibration energy to the webs simultaneously while each of the anvils faces the first or second ultrasonic horn, and wherein the pair of anvils (11, 12) are provided symmetrically with each other with respect to an axis line of the anvils roller; characterized in that the welding system further comprises (b) a velocity-changing device for increasing and decreasing a moving velocity of the webs, wherein the moving velocity of the webs at the first and second gaps when the first and second ultrasonic horns apply the vibration energy to the webs is set smaller than that at a position where the webs enter into the velocity changing device and/or that at a position where the webs exit from the velocity-changing device. (a) the ultrasonic welder comprises: an anvil roller including a pair of anvils (11, 12); a first ultrasonic horn and a second ultrasonic horn that apply vibration energy to the webs in cooperation with the pair of anvils; and carrying means for carrying the webs so that the webs pass a first gap between the anvil roller and the first ultrasonic horn and then pass a second gap between the anvil roller and the second ultrasonic horn, wherein the first and second ultrasonic horns are arranged so that one anvil of the pair can face the first ultrasonic horn at the same time as the other anvil of the pair faces the second ultrasonic horn, wherein the first and second ultrasonic horns apply the vibration energy to the webs simultaneously while each of the anvils faces the first or second ultrasonic horn, and wherein the pair of anvils (11, 12) are provided symmetrically with each other with respect to an axis line of the anvils roller; characterized in that the welding system further comprises an anvil roller including a pair of anvils (11, 12); a first ultrasonic horn and a second ultrasonic horn that apply vibration energy to the webs in cooperation with the pair of anvils; and carrying means for carrying the webs so that the webs pass a first gap between the anvil roller and the first ultrasonic horn and then pass a second gap between the anvil roller and the second ultrasonic horn, wherein the first and second ultrasonic horns are arranged so that one anvil of the pair can face the first ultrasonic horn at the same time as the other anvil of the pair faces the second ultrasonic horn, wherein the first and second ultrasonic horns apply the vibration energy to the webs simultaneously while each of the anvils faces the first or second ultrasonic horn, and wherein the pair of anvils (11, 12) are provided symmetrically with each other with respect to an axis line of the anvils roller; characterized in that the welding system further comprises (b) a velocity-changing device for increasing and decreasing a moving velocity of the webs, wherein the moving velocity of the webs at the first and second gaps when the first and second ultrasonic horns apply the vibration energy to the webs is set smaller than that at a position where the webs enter into the velocity changing device and/or that at a position where the webs exit from the velocity-changing device.