Provided is a seat which comprises: a seat frame constituting a framework for a seat cushion or seat back; a synthetic resin hook member having a locking portion locked to a frame member as a part of the seat frame; a stretchable planar elastic body attached to the hook member locked to the frame member so as to be mounted to the seat frame in a tensioned state; and an anti-expansion mechanism for avoiding a situation where the hook member is expanded and thereby the hook member is disengaged from the frame member.