PURPOSE: To precisely preform a folding position by a method wherein a flap of a packaged body with gusset is folded down so that the flap is brought into contact with a wrapping film surrounding the packaged body to bond a part of contact point and thereafter the flap is held and transferred to heat it, thereby causing the flap to be preformed into a folded state.

CONSTITUTION: When a packaged body 5 with gusset being transferred by a conveyor is transferred into a folding device, an action rod 26 moves to the right and oscillates film restraining plates 21 inwards (on the side of packaged body 5) so that film parts 3a are held between the plates 21 to bring the parts 3a into close contact with the body to be package$ Next, cylinders 31 are actuated to lower film rollers 35 so that flaps 4 are folded downward. At the lowermost positions of the rollers 35, cylinders 41 are actuated to advance spot sealers 42, thereby thermally bonding the parts 3a to the flaps 4, following which each of the parts is returned to the original positions and then the body 5 is taken out of the folding device.