To install a panel in a ceiling joist without screwing, to take away any panel, with all the panels are installed in place, and to put the panel taken away back o the former state.

In panels 11, which are formed into two pieces parallel with each other, an extended part 19A of the panel 11 on one side is mounted on a panel receiving part 27 of one ceiling joist 9, while an extended part 19C of the panel 11 on the other side is mounted on the panel receiving part 27 of the other adjacent ceiling joint 9, so that the panels 11 are supported, with a going-down part 21 dropped into a recessed part 29 of the ceiling joist 9, In the panels 11 neighboring on each other, respective extended parts 19A and 19C are overlapped with each other on the panel receiving part 27 of the ceiling joist 9. A spring 35 is interposed between the extended parts 19A, 19C, both of which are mounted on the panel receiving part 27, and a spring end locking part 31 of the ceiling joist 9. Owing to the resiliency of the spring 35, the extended parts 19A, 19C of the panel 11 are pushed against the panel receiving part 27 of the ceiling joist 9. By lifting the panel 11, resisting the resiliency of the spring 35, the panel can be taken away.