While naturally suctioning air between a first thermoplastic resin sheet (20) and a second thermoplastic resin sheet (21) from a fluid passage (23) formed in a first thermoplastic resin sheet (20), the first thermoplastic resin sheet (20) is subjected to vacuum-molding by a first vacuum molding die (2), a first shaping section is formed in the first thermoplastic resin sheet (20) by a molded section (5) of the first vacuum molding die (2), the second thermoplastic resin sheet (21) is subjected to vacuum-molding by a second vacuum molding die (3), a second shaping section is formed in the second thermoplastic resin sheet (21) by the molded section (11) of the second vacuum molding die (3), and a hollow section (300) is defined between the first thermoplastic resin sheet (20) and the second thermoplastic resin sheet (21) by the first shaping section and the second shaping section.