To reduce an equipment cost as compared to a high frequency heating method, to simplify an operation, to improve heat efficiency as compared to a hot air blowing heating method and to suppress worsening of a working environment.

A thermosetting type adhesive 6 material is filled/sealed between the end parts of an outer plate 2 and the inner plate 4, the end part of the outer plate 2 is held back in the surface direction of the inner plate 4, and a hemming part 8 is formed. For the hemming part 8, a pair of heating members 10a, 10b, in which the surface temp. are controlled to 300°C, are brought in contact with the hemming part 8 for ≥5 sec so as to hold from up/down, the hemming part is heated to tentatively harden the adhesive material 6.