PURPOSE: To surely install a mat in simple manners by supporting an engaging piece for an installation hole edge member on the mat side in turn-switching manners between an engagement attitude and a demounted attitude through a main shaft, onto a supporting pillar erected onto a bracket on a car body side.

CONSTITUTION: A device body is constituted by combining a bracket 1 fixed onto a car body side and an installation hole edge member 5 on a mat M side which is engaged in demountable manners onto the bracket. In this case, a supporting body 2 is erected onto the bracket 1, and an engaging piece 4 is supported onto the supporting body 2 in turn-switching manners between an engagement attitude 4 and a demounted attitude B through a main shaft 3. As for the engaging piece 4 in the engagement attitude A, the both edge parts 4a and 4b project from the top surface 2a of the supporting body 2, and in the demounted attitude B, the whole is formed so as to not to project from the top surface 2a of the supporting body 2. Further, the engaging piece 4 is switched from the demounted attitude B to the engagement attitude A by an automatic engaging means 7. Further, the engaging piece 4 tilts at a release suppressing attitude C, and the release of the installation hole edge member 5 is suppressed.