motions of the movable roof members 24 and 25 may be performed since the widths as well as the horizontal angles of the movable roof members 24 and 25 are designed to be consistent in this embodiment the central and side movable roof members 24a 24b 25a and 25b simultaneously reach the right most or left most wing if the central movable roof members 24a and 25a are driven at a angular velocity which is double that of the right or left side movable roof members 24b and 25b if the angular velocities of all the roof members 24 and 25 are the same first the relative location of the central and the side roof members 24a and 24b or 25a and 25b are kept as in the closed position when travelling however the side movable roof members 24b and 25b reach the side most wings 52 or 54 before the central movable roof members 24a and 25a then the central movable roof members 24a and 25a travel alone until they reach the side most wings 52 or 54 with the above construction the configuration of the movable roof members 24 and 25 and the travelling mechanisms including the tracks 40 42 44 and 46 and the electrically driven wheels 48 for travelling the members can be simplified and made compact accordingly the costs of constructing the roof members and the travelling mechanisms is decreased in addition since the movable roof members 24 and 25 are stored under the fixed roof members 26 when the roof 20 is open the portion of the opening is broader than that of conventional openable roofs this heightens the sense of openness and freedom experienced by the audience in the stadium 22 additionally since the movable roof.