To protect an infant on a child seat of an assistant's seat from pressure caused by the expansion of an air bag.

A seat cushion 2 of a assistant's seat 1 is formed to be pulled up forward, and the lower face of the seat cushion 2 is provided with a protecting member 23 displaceable into a storage state disposed along the lower face of the cushion and an upward sprung-up pulled-out state. An air bag 28 at the time of expansion is hindered from entering into an occupant restricted are by a shield member 23 in the pulled-out state so as to protect an infant placed on a child seat 29.