To stably maintain the base isolation function over a long period by preventing condensation by avoiding contact between a sliding surface of a smoothing plate and an inner surface of a protective cover.

This protective cover is arranged between a lower skeleton 7 and an upper skeleton 8, and covers a sliding base isolation device for mutually arranging the smoothing plate 2 arranged on the lower skeleton side and a sliding material 3 of an elastic support part arranged on the upper skeleton side by contacting the respective sliding surfaces. The protective cover is a cross-sectional view box-shaped mold having a lower opening part covering the smoothing plate and the sliding material, and a columnar body 10 for preventing contact with the smoothing plate is arranged on an inside ceiling surface of a cross-sectional view box shape.