PURPOSE: To eliminate a pressure on a back temporarily in a simple constitution by disposing a plurality of rectangular elevation parts, providing endless track means on both side parts inside a frame of a bed, and driving the rectangular elevation parts through elevation arms of rotary shafts of pinions disposed in their racks.

CONSTITUTION: A plurality of rectangular elevation parts 4 driven to elevate are disposed in a mattress 3 mounted on a frame 2 of a bed. Endless track means 5 inside the frame 2 are actuated to drive chains 7, engage racks 8 with pinions 10 in order, stand interlock arms 11, move up elevation arms 12, and protrude the rectangular elevation parts 4 from the mattress 3. When the pinions 10 is released from the rack 8, the rectangular elevation part 4 rotates the elevation arm 12 further by its own weight, and the elevation arm 12 is applied to a stop part 14 at the interlock arm 11, so the rectangular elevation part 4 is held at the position. The rectangular elevation parts 4 are thus protruded from the mattress 3 in order.