To place an elastic body so as not to cause rattle.

Engaging holes 6 are arranged in the neighborhood of four corners of a bottom wall 5a of an open pocket 5, each of engaging projected parts 8/9 to be respectively a pair on the front side and the rear side is integrally provided at a position corresponding to a lower surface of a mat 7 placed on the bottom wall, flat surface parts 8a/9a confronting each other are formed on the engaging projected parts to be a pair, projected line parts 8d/9d are projected and provided on the flat surface parts, and a relation between length c1/c2 between the flat surface parts and length b1/b2 between the projected line parts and length a1/a2 between most approaching parts of each of a pair of the engaging holes 6 is specified as c1alb1(c2a2b2). A pair of the engaging projected parts is elastically deformed each other in a direction to be separated from each other in a state inserted into the engaging holes, it is possible to eliminate the rattle in a direction to pass through a pair of the engaging projected parts by its elastic restoring force, and it is possible to prevent the rattle in a state where the elastic body is placed by a simple structure of providing the engaging projected parts integrally on the elastic body and providing the engaging holes on an interior member.