To facilitate installation of a backrest part for a rear seat by allowing elastic deformation in installation of a locking member to a journal and inserting the locking member into a throat perpendicularly to the axial line of the journal.

First of all, a journal 9 is radially inserted into a locking member 19 via a slot 25. As a central part 21 has resiliency, the slot 25 can be widened sufficiently. Subsequently, when seat backrest parts 1, 1' are arranged on a support 13 and pressed down, the locking member 19 starts entering the inside of a throat 17. Lips 23 are brought close to each other when they pass through a shoulder part. As the lips 23 have great rigidity, mutual approach of the two lips 23 is allowed mainly by existence of the slot 25 and resiliency of the central part 21. When the central part 21 abuts to the bottom part of the throat 17 after finish of insertion, the upper ends 27 of the lips 23 are snapped on the lower side of the shoulder part. That is, the backrest part can be installed because of resiliency of the central part 21 and a play based on elastic deformation of the lips 23.