To enable a perfect rust-proofing treatment to be performed on the internal surface of a metal container made of iron for a fire extinguisher by completely releasing air in the metal container to the outside when a corrosion-resistant bag body is fitted in the metal container and expanded.

A gas releasing hole communicating the internal surface with the external surface is drilled on the metal container in advance, and at the same time, the corrosion-resistant bag body is fitted in the metal container, and then, the bag body is expanded to be closely joined to the total surface of the internal surface of the metal container. Thus, when the corrosion-resistant bag body is expanded, air in the metal container is discharged to the outside of the metal container from the gas releasing hole accompanying the expansion of the bag body. As a result, the unfavorable joining of the bag body extending to the total surface of the metal container internal surface resulting from the remaining of air in the container is eliminated, and as the container of the fire extinguisher, an inexpensive metal container made of iron can be used. In addition, the function as the fire extinguisher is not spoiled even after being set for a long period of time, and the durability can greatly be increased.