Provided is a bottom for a bed device that relieves the sensation of pressure during an elevating operation to lift the back, and that at the same time is lightweight and easy to transport, as well as being easy to clean. This bottom for a bed device has a back lifting mechanism and is provided with a flexing bottom capable of flexing, situated between a bottom below the back and a bottom below the hips. During a back lifting operation, the flexing bottom flexes between the bottom below the back and a bottom below the hips, while sliding downward across the surface of the uplifting bottom below the back.
