To provide a manufacturing method of a cell pack without damaging parts like cell, and to provide a welding device used for the manufacture of the cell pack.

For the manufacturing method of the cell pack, inserting the cell in a case with an opening, and welding a cover to the case by adding a load to the cover with an ultrasonic vibration of a horn in a state of closing an opening of the case by a cover, the cover is welded after pushing the cover toward the case in a state to close the opening of the case by the cover. For the welding device comprising an ultrasonic generator 5, a horn 4, connected to the ultrasonic vibrator and adding a load to the objects to be welded by moving in a prescribed direction, and a control means 7 controlling the driving of the ultrasonic generator 5 and the movement of the horn 4, the control means 7 has a control circuit or a program which makes the horn 4 add a load to the objects to be welded without moving, and afterwards, add a load to the object to be welded by making the horn 4 ultrasonically vibrate.