To mount two kinds of electronic circuit boards as a small card and a two-stage high card without attaching and removing a front guide rail.

In this mounting shelf on which an electronic circuit board is mounted, the mounting shelf 6 is provided with guide rails 1, 2, 3 in the upper stage, the middle stage and the lower stage. A small card 4 is mounted between the guide rails in the respective stages. In addition, a two-stage high card 5 is mounted between the guide rail 1 in the upper stage and the guide rail 3 in the lower stage so as to cross the guide rail 12 in the middle stage. The guide rail in every stage is formed in such a way that a fixed-side rear-part guide rail 12 is combined with moving-side front-part guide rails 11. The front- part guide rails 11 are held by the rear-part guide rail 12 so as to be slidable. They are pulled out from the open edge of the rear-part guide rail to its extension direction so as to hold the small card 4, and they are housed within the range of the length of the rear-part guide rail so as to hold the two-stage high card 5.