A mattress including a plurality of cells aligned and disposed on a body pressure working surface of a substrate that supports a human body; and a pressure control member that changes a height of each cell by adjusting a pressure in a fluid chamber formed within the cell. The cells are configured such that as the each cell is inflated by fluid supplied to the fluid chamber within the cell, the height of the each cell increases and a width dimension of the each cell in a direction of alignment decreases, and that peripheral portions of adjacent cells in the direction of alignment are overlapped with each other prior to cell inflation.