To facilitate reduction of cost and installation work by burying a box nut to engage with a hexagonal bolt by projecting upward from a mat surface on one surface of a rubber mat and connecting a projected part to a bottom part of a concrete caisson.

A box nut 2 is buried on one surface side of a rubber mat 1 by opening its inner surface, and a head part of a hexagonal bolt 3 is projected upward from a mat surface by screwing the hexagonal bolt 3 on each of the box nuts 2. Thereafter, a plural number of the rubber mats 1 are spread and surrounded by a formwork, and the head part of the hexagonal bolt 3 is connected to a bottom part 4 of a concrete caisson by depositing concrete. Consequently, it is possible to carry out installation construction with little dispersion regardless of rain or sunshine by eliminating troublesome installation work.