To discriminate whether to be fixed in a normal position by visual observation after installation, by simplifying installation work of a loudspeaker, a monitoring camera and a radio antenna device to the thin plate ceiling of a commercial building.

A hole 3 is opened in the thin plate ceiling 2. In the loudspeaker, the monitoring camera and the radio antenna device 1 installed in the ceiling, respective apparatuses are provided on a base 4. A plurality of fixing screw holes 8 and a rotary groove hole 9 of an angle caused by rotation around the fixing screw holes 8, are arranged in the base 4. A fixing piece 10 is provided in a fixing screw 7, and a metal fitting part 10b bent in a crank state of the fixing piece 10 is inserted into the rotary groove hole 9. The fixing piece 10 is rotated around the fixing screw 7, and is made to enter all the way onto the thin plate ceiling 2, and the base 4 is fixed to the thin plate ceiling 2 by fastening the fixing screw 7. The fact that the fixing piece 10 exists in the normal position can be confirmed by visual observation in a position of a part where the metal fitting part 10b bent in the crank state of the fixing piece is exposed from the rotary groove hole 9.