To prevent pressure to the belly part, etc., of a user at the time of sitting-up by juxtaposing multiple band-shaped support plates towards a mobile direction in a mobile bottom and permitting the sliding of a matless placed on it to be excellent.

A back bottom 1 being the mobile bottom is constituted by disposing and juxtaposing multiple support plates 10 in a frame material consisting of pipe-made side frame materials 11 and 11, a rotary shaft 14 supported by the bracket of the bed frame 6 in both ends so as to be freely rotatable and a front frame material 15 with an almost square cross-section. Then, the respective support plates 10 are band-shaped metallic plates and provided in the up and down direction of the back bottom 1 and the both ends are fixed to the rotary shaft 14 and the front frame material 15. When the back bottom 1 is erected by a motor, push-down direction force is actuated on the user together with the matless on the back bottom 1. Since the back bottom 1 is provided with structure where the matless is easily slid, the push- down force of the back bottom 1 is not actuated on the user on the matless so that the belly part is not pressurized when an upper body is erected.