A mattress comprising: a plurality of cells (16, 60) arranged on a body pressure working surface of a bottom mat that supports a human body, a pressure control means that changes a setting of a height of each cell (16, 60) by adjusting a pressure in a fluid chamber formed within the cell (16, 60), and an elastic cushion layer provided on a body pressure working surface of the cells (16, 60), being characterized in that: a constricted portion is formed in a middle in the height of the each cell (16, 60) so that the body pressure working surface of the cell (16, 60) is configured to be tilted following deformation of the cushion layer as a result of the cells' precession-deformation at the constricted portion, each cell (16, 60) is structured as a combination of an upper bag portion and a lower bag portion, each cell (16, 60) is supported by the bottom mat only at the center portion where a port is provided, the lower bag portion is allowed to tilt centered around the port at the center of the bottom surface by virtue of the deformation of the lower bag portion and the bottom mat, the tilt angle of the upper surface of each cell (16; 60) is the sum of the tilt angle of the upper bag portion against the lower bag portion and the tilt angle of the lower bag portion against the bottom mat. a plurality of cells (16, 60) arranged on a body pressure working surface of a bottom mat that supports a human body, a pressure control means that changes a setting of a height of each cell (16, 60) by adjusting a pressure in a fluid chamber formed within the cell (16, 60), and an elastic cushion layer provided on a body pressure working surface of the cells (16, 60), being characterized in that: a constricted portion is formed in a middle in the height of the each cell (16, 60) so that the body pressure working surface of the cell (16, 60) is configured to be tilted following deformation of the cushion layer as a result of the cells' precession-deformation at the constricted portion, each cell (16, 60) is structured as a combination of an upper bag portion and a lower bag portion, each cell (16, 60) is supported by the bottom mat only at the center portion where a port is provided, the lower bag portion is allowed to tilt centered around the port at the center of the bottom surface by virtue of the deformation of the lower bag portion and the bottom mat, the tilt angle of the upper surface of each cell (16; 60) is the sum of the tilt angle of the upper bag portion against the lower bag portion and the tilt angle of the lower bag portion against the bottom mat. a constricted portion is formed in a middle in the height of the each cell (16, 60) so that the body pressure working surface of the cell (16, 60) is configured to be tilted following deformation of the cushion layer as a result of the cells' precession-deformation at the constricted portion, each cell (16, 60) is structured as a combination of an upper bag portion and a lower bag portion, each cell (16, 60) is supported by the bottom mat only at the center portion where a port is provided, the lower bag portion is allowed to tilt centered around the port at the center of the bottom surface by virtue of the deformation of the lower bag portion and the bottom mat, the tilt angle of the upper surface of each cell (16; 60) is the sum of the tilt angle of the upper bag portion against the lower bag portion and the tilt angle of the lower bag portion against the bottom mat.