To include a lower foundation 30 provided on a ground 7 and an upper foundation 31 provided above the lower foundation 30 with a base isolating device 32 interposed therebetween. The upper foundation 31 includes a bottomed steel-plate form 40, a hull structure 41 arranged in the steel plate form 40, and a concrete part 46 filled in the steel plate form 40. The hull structure 41 is constituted in a lattice shape by a plurality of transverse reinforcing ribs 42 that extend on a bottom steel plate 40a of the steel plate form 40 and a plurality of longitudinal reinforcing ribs 43 that extend on the bottom steel plate 40a of the steel plate form 40 so as to be orthogonal to the transverse reinforcing ribs 42.