A flexible film of a synthetic resin is arranged on a rigid support plate, and both edges of the film in the tensely extending direction are attached to fixing portions of the support plate. The support plate is fitted into a frame body via the film, so that the film is tensely extended and supported between the frame body and the support plate. The support plate has arcuate raised portions extending in parallel to the fixing portions on the side of the film. In a case where the film is arranged so as to extend along the support plate to be attached to the fixing portions, the film is pushed by the arcuate raised portions of the support plate, so that a gap is formed below the film. When the support plate is fitted into the frame body via the film, the film is pushed and extended by the angular portions of the frame body, so that the gap is stopped.