A seat device includes a seat part that is capable of changing a posture to a use posture that allows sitting on the seat part and to a non-use posture that does not allow sitting on the seat part. A seat support can support the seat part in a state in which the seat part has been changed to the use posture. A guide member has a free groove that slidably guides a sliding part of the seat support and a regulation groove that regulates sliding of the sliding part in a state in which the seat part has been changed to the use posture. The guide member can be fixed on the seat part. A spring biases the seat support so that the sliding part is held in the regulation groove in the state in which the seat part has been changed to the use posture. A lever may cause the sliding part to slide from the regulation groove to the free groove and allow the seat support to swing downward.