In a conventional support structure where the position for supporting a foot bottom end can be set to high and low positions with a knee bottom raised, a change from the high position to the low position can be made only when the knee bottom is raised and, when the knee bottom is lowered, the setting is cancelled and set to the high position. One end side of the knee bottom (2) is pivotally and liftably supported on a bed frame (1), one end side of a foot bottom (6) is pivotably connected to the other end side of the knee bottom (2), a support stay (7) is pivotaby connected to the lower side on the other end side of the foot bottom, support means (11) of a support section (9) formed on the front end side of the support stay (7) is connected to the bed frame. The support means is constructed from a guide section (12) and an engagement section (13) for movably supporting the support section of the support stay. The guide section can support the support section that moves as the knee bottom pivots in its pivot range. The engagement section is provided above or below the end section side of the guide section where the support section is located when the knee bottom lowers.