To provide an optional seat.

An optional seat comprises: at least one seat body blow-molded from a reinforced plastic; and a member such as a ventilation device set. The optional seat is applied to a carry case. A shape of the seat body can vary according to a different sitting posture of a user. The seat body is developed and is shaped by charging/discharging air into/from the seat body, or is exhausted to be stored. The seat body can be utilized as an individual solid, is connected to the ventilation device set of the carry case to be utilized, and is stored inside a storage space of the ventilation device set. When using the optional seat, the ventilation device set ventilates the inside of the seat body, and the seat body is developed and is shaped, so that the optional seat has convenience allowing optional sitting.