A control unit controls a drive unit so that a back angle ±, which is an elevation angle of a back bottom, and a knee angle ², which is an elevation angle of a knee bottom, change according to a pre-set pattern. The control unit comprises a memory unit for storing a plurality of patterns of linking a coordinate point (0,0) in a (±,²) coordinate, in which each of the bottoms is in a horizontal state, and a coordinate point (± 0 , ² 0 ), in which the back bottom is raised, with a plurality of points a selection unit for selecting one pattern from the plurality of patterns stored in the memory unit and a computation unit for controlling the drive unit so that the back angle ± and the knee angle ² change according to the pattern selected by the selection unit. Therefore, the patient can be prevented from being displaced on the bed and from being subjected to a sense of pressure on their stomach or chest.
