A cushion includes a first cushion portion, a second cushion portion, and a connecting portion interconnecting the first cushion portion and the second cushion portion such that the first cushion portion and the second cushion portion are brought to an open position to unfold the cushion. The first cushion portion and the second cushion are movable to a closed position to fold the cushion along the connecting portion into a generally rectangular parallelepiped. The connecting portion is provided along one corner of the generally rectangular parallelepiped and interconnects one end of a first surface of the first cushion portion and one end of a first surface of the second cushion portion. An inclining surface of the first cushion portion and an inclining surface of the second cushion portion are both planar surfaces.