To solve a problem that the releasing work of the new strain generated when a skeleton is welded must be conducted after a block is completed or mounted because thin panels are welded together before the skeleton such as a girder is welded.

A channel member 5 is stretched and fitted at the plate end section in advance together with longitudinals 2 along a plate joint section 3 on one or both of a plurality of thin panels 1 fitted with the longitudinals 2 in parallel with the plate joint section 3 in advance, and a plurality of thin panels 1 are installed at a plate clearance margin G on a surface plate so that the channel member 5 is located astride the adjacent panel 1. The skelton 6 is arranged astride the thin panels 1 in the direction perpendicular to the longitudinals 2 then welded to the thin panels 1 while the plate joint section 3 is kept unwelded. A thin panel block B is inverted, the thin panels 1 are adjusted in a flat state on the surface plate to release the strain generated by the welding of the skelton 6 through the clearance margin G, then the channel member 5 is welded to both thin panels 1 to connect the thin panels 1 together.