To dispense with removal of existing gable walls and secure the original aseismetic strength, by arranging a large-sized rigid frame of a reinforced concrete structure with buit-in steel frames having large openings at both gable faces of a building and removing walls covering the large openings.

Gable walls 5 are arranged at both ends of the ridge direction respectively and a plurality of intermediate frame structures 6 are arranged between both gable walls at every specified distance. These gable walls 5 are provided with aseismatic walls at both lower and upper building parts 2, 4 and constituted of steel encased reinforced concrete structures. And columns and beams 7 made of H-steels or the like are used for the steel frame. In the columns and the beam frames 7 of the upper building part 4 in particular, large-sized rigid frames 9 are constituted of girders 8 laid between the upper ends and large openings are formed at the central part. All parts except the large openings 10 are reinforced by brace frames made of H-steels or the like. If an extensive building is constructed afterward, the large opening 10 is constituted of curtain walls for instance so that it can be easily removed even afterward.