To eliminate a need for a molding jig for exclusive use for a rubber bag, to facilitate handling of a rubber sheet, and to remarkably save a manufacturing time and cost for the rubber bag by a method wherein a plurality of rubber sheets are mounted on the whole of the inside surface of the insulation inside a motor case through a film for peel, and the rubber bag is integrally molded at the inside of the motor case by using the whole of the rubber sheet.

An insulation 2 is mounted on the whole of the inside surface of a motor case 1 for as rocket motor and after a separation film 3 is mounted on the whole of the inside surface of the insulation 2, a number of rubber sheets 4 molded in a given size are mounted on the whole of the inside surface of the separation film 3 to form a rubber bag G for pressure resisting test by using the whole of the rubber sheet. This constitution saves a manufacturing time and cost, and eliminates displacement therebetween through integral formation of the rubber bag G to the motor case 1.