A control unit controls a drive unit so that a back angle α, which is an elevation angle of a back bottom, and a knee angle β, 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 (α,β) coordinate, in which each of the bottoms is in a horizontal state, and a coordinate point (α0, β0), 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 α and the knee angle β 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.