To easily perform maintenance of a substrate even if a component is disposed by the side of the substrate.SOLUTION: A lower substrate 14, an upper substrate 22 and a connection piece 34 are used and a notch 28 is formed on the lower substrate 14. The upper substrate 22 is disposed in parallel with the lower substrate 14 while being spaced apart therefrom, and a through hole 36 is formed therein. The connection piece 24 connects the lower substrate 14 and the upper substrate 22. A portion of one end of the connection piece 24 is caused to enter a side face of the notch 28 on the lower substrate 14 and upward vertically connected to a U-shaped metal fitting 26 disposed on a bottom face of the lower substrate 14. One end of the connection piece 24 is caused to pass the through hole 36 and protrude to the side of the upper substrate 22, and the other end of the connection piece 24 is bent in a direction opposed to the upper substrate 22, screwed to the second substrate from above and connected to the U-shaped metal fitting 38 on the upper substrate 22.SELECTED DRAWING: Figure 4