When pressure detected by a pressure sensor connected to a calibration standard cell has continuously fallen within a regulated pressure range at calibration for a regulated duration at calibration, zero-adjustment is performed based on the pressure value detected by the pressure sensor. Then, communication between the plurality of cells is created to perform internal pressure adjustment of the plurality of cells to a predetermined internal pressure value, based on the pressure value detected by the pressure sensor connected to the calibration standard cell and calibrate each of pressure sensors connecting to the plurality of cells, based on the adjusted internal pressure. With this scheme, it is possible to a mattress and the like that can perform zero-point adjustment efficiently by adjusting the zero point in the same manner even if the mattress is formed of a plurality of branches, without releasing the pressure sensor of each cell to atmospheric pressure.