To provide a closing structure of a base isolation clearance section by which opening parts on both sides formed by the base isolation clearance section of an entrance porch or the like are closed successfully to impart a good appearance.

A pair of planar closing members 7, 7 closing the base isolation clearance section 6 from both sides are placed to bridge the clearance between a base-isolation side and a non-base-isolation side. The planar closing members 7, 7 are connected to each other by a spring 8 so that a spring force may be applied in the direction of putting them closer to each other. Each of the planar closing members 7, 7 is provided with an engaging part 9 on an inner surface of the side edge part extending to a lower side 4 of the entrance porch in a manner that the engaging part 9 is engaged with the lower side 4 of the entrance porch to prevent displacement in an in-surface direction. The engaging part 9 is formed to have an inclined plane 9a which works to prevent breakage by opening the pair of planar closing members 7, 7 in an out-surface direction when the base-isolation side and the non-base-isolation side are relatively displaced in a dierction of coming closer to each other upon seismic isolation.