A control amplitude and a control frequency of a voltage applied to a piezoelectric pump are determined, control is carried out so that a voltage at the determined control amplitude and control frequency is applied to the piezoelectric pump, and a blood pressure value is calculated based on a cuff pressure detected by a pressure detection unit during inflation when the cuff pressure is increased by the piezoelectric pump. The amplitude of the voltage is controlled in predetermined steps, and a voltage having an amplitude that is a value above the control amplitude by at least one step and a voltage having an amplitude that is a value below the control amplitude by at least one step are applied, in a predetermined order, so that the control is essentially the same as when a voltage at the determined control amplitude is applied.
