In the mattress (10), a plurality of cells (16) are aligned and disposed on the body-pressure acting surface of a bottom mat (20) as the substrate supporting a human body, and pressure control means (78) is provided that alters/sets the height of the cells (16) by adjusting the pressure of the fluid chambers (60) of the cells (16). In accordance with the cells (16) expanding as a fluid is sent into the fluid chambers (60) of the cells (16), the height of the cells (16) rises and the width dimension in the direction of the alignment of the cells (16) decreases. Meanwhile, before the expansion of the cells (16), the peripheral portions (64) of adjacent cells (16) in the direction of alignment are caused to overlap each other.
