An object of the present invention is to provide a welding system wherein the positional accuracy of welding and the quality of welding are improved. The present welding system includes an ultrasonic welder for welding a plurality of webs W together while carrying the webs W. The ultrasonic welder includes: an anvil roller 10 including a pair of anvils 11, 12; a first ultrasonic horn 21 and a second ultrasonic horn 22 that apply vibration energy to the webs W in cooperation with the pair of anvils 11, 12. The pair of anvils 11, 12 are provided symmetrically with each other with respect to an axis line O of the anvils roller 10. The first and second ultrasonic horns 21, 22 apply the vibration energy to the webs W simultaneously while each of the anvils 11, 12 faces the first or second ultrasonic horn 21, 22.