PROBLEM TO BE SOLVED: To provide a new method for improving measurement accuracy of blood pressure.SOLUTION: An ultrasonic sphygmomanometer 1 includes a pressurizing unit 30 for applying external pressure to a region to be measured. A pressure detection unit 160 detects the pressure applied by the pressurizing unit 30. A pressure control unit 150 controls the pressurizing unit 30. A differential pressure calculation unit 145 calculates a differential pressure between internal pressure and external pressure through the use of a first pressure and a second pressure detected by the pressure detection unit 160 in a first state and a second state which have a different pressure difference between internal pressure and external pressure in a blood vessel to be measured. A blood pressure estimation unit 140 estimates the blood pressure of a subject through the use of a blood vessel diameter difference between first blood vessel diameter and a second blood vessel diameter measured by a blood vessel diameter measurement unit 110 in the first state and the second state, a correlation expression 821 indicating correlation characteristics between the blood pressure and the blood vessel diameter of the blood vessel to be measured, and the differential pressure calculated by the differential pressure calculation unit 145.
