PURPOSE: To make an opening rate adjustable by dividing an intermediate roof in the diametric direction, so that a division line between a fixed roof and the intermediate roof is crossed with a division line of the intermediate roof in its diametric direction in the outside of a structure, and moving the intermediate roof to freely run.

CONSTITUTION: Travelling tracks 3a, 3b for moving movable intermediate roofs 1a, 1b are arranged in a structure formed so as to surround an open ground 9. Both end fixed roofs 2a, 2b are formed in such a manner that their division lines with the intermediate roof are crossed with a division line between the fellow intermediate roofs in the outside of the structure. The movable intermediate roofs 1a, 1b are mounted on intermediate roof saddles 4a, 4b and moved along the travelling tracks 3a, 3b. In this way, an opening rate of the roof can be adjusted in accordance with an event.