To provide a back lifting mechanism for reducing pressure on a body or a deviation which is generated in lifting the back of the body by adjusting the back lifting mechanism in response to the movement of the back part of a lying person when a bottom corresponding to the back is lifted concerning a bed having a lifting function.

An arm rotation shaft 2 fixed to the main frame 1 of the bed is connected to a back bottom rotation shaft 6 attached to a back bottom 5 with the use of expansible arms 3a, 3b, so as to lift the back by rotating the arm 3a with the arm rotation shaft 2 as a center in the bed. The bed includes the mechanism for enlarging a distance between both the rotation axes by allowing the arm 3b to move in the opposite direction of the arm rotation shaft 2 along the arm 3a together with the rotation of the arm 3a. A position to attach the back bottom rotation shaft 6 to the back bottom 5 and speed to enlarge the distance between the arm rotation shaft 2 and the back bottom rotation shaft 6 in lifting the back are adjusted in response to a body type, a lying position, and the thickness of a mattress, etc.