To improve self-supporting properties and shape retaining properties of a self-supporting bag (standing pouch) with a bottom gusset system.

A non-fused part 3 extending in the up and down direction is formed between laminated films constituting a surface and a back face on a side edge part 2 of the self-standing bag 1. The non-fused part 3 has a closed contour, and a notch 4 communicating with the inside of the non-fused part 3 and the outside of the bag is formed on the laminated film of the non-fused part 3. An air blowing nozzle 17 is applied to the notch 4 to blow a compressed air into the inside of the non-fused part 3 from the notch 4. Under this condition, a lower part of the notch 4 is fused and sealed by pinching it between hot plates (a sealed part 12), and the compressed air is sealed in the non-fused part 3. The non-fused part 3 sealed with the compressed air (an air bag) improves the self-supporting properties and the shape retaining properties of the bottom gusset system.