To provide a base isolation device which is installed on a contacting surface between a construction and the ground and is composed of a steel material including a sliding slit on its one end with an upper lid of a steel ball whose upper surface is bearing-treated as a support point, a T-shaped sliding shaft with a buffer spring on its top and a receptacle for a foundation column of a construction on its other end.

A horizontal quake shock due to an earthquake to the construction is totally absorbed by the steel ball 1 with the bearing-treated upper lid placed on the portion A of the steel material 3. Furthermore, the construction being supported by the receptacle at the end portion B of the steel material 3 which has the steel ball as a supporting point, an up-and-down shock is totally offset and thus a shock toward the construction is completely absorbed by the spring which is installed on the end portion of the steel material C of a tread-mortar type for shutting down any transmission of the up-and-down shock toward the construction.