A backrest portion supported, such as to be pivotable around an axis extending along its width direction center line, by a base, a left arm holding portion projecting forward from a left side portion of the backrest portion and supported, such as to be pivotable up and down around its base end portion, by the backrest portion, a right arm holding portion projecting forward from a right side portion of the backrest portion and supported, such as to be pivotable up and down around its base end portion, by the backrest portion, a left leg holding portion extending downward from a vicinity of a left side portion of a front edge of a seat portion at a front side of the base and supported, such as to be pivotable up and down around its upper end portion, by the base, and a right leg holding portion extending downward from a vicinity of a right side portion of the front edge of the seat portion at the front side of the base and supported, such as to be pivotable up and down around its upper end portion, by the base are included.