To provide an interexchangeable seat comprising the features such as a booster seat, a child seat, a armrest and the like.

A child booster seat cushion 34 is extended forward in its use position spaced above a seat cushion 12, and can be used as a booster seat cushion 34 for child (elevate) for elevating a child to a position in which an adult restraint systems 20, 22, 24 can be used. A seat back 14 has a movable panel 30, which covers a recess 32 of the seat back 14 accommodating the booster seat cushion 34. When the booster seat cushion 34 is moved to its use position, the panel 30 is returned to its close position, whereby the panel 30 forms the seat back face 18 for both adult and child. The booster seat is removable from the seat back, and replaceable with another integral child seat unit or an armrest unit having unique features.