Bag making method

A bag making method comprising:

• folding a web to form a continuous first panel part, a continuous second panel part, and a continuous gusset part from the web, the gusset part being folded and interposed between the first and second panel parts;
• sandwiching and compressing the first panel part, the second panel part, and the gusset part folded and interposed between the first and second panel parts, with a horn and an anvil, the horn having a facing surface that faces the anvil, the anvil having a facing surface that faces the horn, at least one of the horn or the anvil including protrusions that protrude from the facing surface thereof;
• ultrasonic-welding a first portion and a second portion of the gusset part to each other using ultrasonic vibration of the horn while the first panel part, the second panel part, and the gusset part are sandwiched and compressed by the horn and the anvil;
• before ultrasonic-welding the first portion and the second portion to each other with the horn and the anvil, heat-sealing the web using a seal unit including a pair of heat seal bars to soften the first panel part, the second panel part, and the gusset part, wherein the ultrasonic-welding is performed in a state in which the first panel part, the second panel part, and the gusset part are softened;
• heat-sealing an ultrasonic-welded region where the first portion and the second portion have been ultrasonic-welded to each other by the horn and the anvil, using a second seal unit including a second pair of heat seal bars, subsequent to the ultrasonic-welding;
• and cross-cutting the web in a width direction of the web across the ultrasonic-welded region so as to make a bag.