PURPOSE: To make it possible to prevent errors in a maximum value of pulsating pressure waves by calculating a measured difference between a cuff pressure and a sampled cuff pressure so as to calculate a crawling reduced pressure value, by calculating an actual difference which is obtained by subtracting the crawling reduced pressure value from the measured difference, by positive actual adding value added with the actual difference during the actual value is maintained to be positive so as to detect the maximum value.

CONSTITUTION: Among five cuff pressures stored in a pressure memory means 21, the oldest cuff pressure is subtracted from the newest cuff pressure by a measured difference calculating means 23 so as to calculate a variation in cuff pressure in an infinitesimal time as a measure difference. A crawling reduced pressure value calculating means 22 calculates a crawling reduced pressure value in an infinitesimal time. An actual difference calculating means 24 subtracts the crawling reduce pressure value from the measured difference so as to calculate an actual difference. A positive actual difference adding means 25 adds all successive positive actual differences to each other if an actual difference for every sampling is positive, and thus added value of the positive actual differences is stored and transmitted to a maximum value detecting means 26. If the content of the maximum value detecting means 26 is greater than 1 while the positive actual difference adding means 25 delivers zero, a value held by the maximum value detecting means 26 is determined to be a maximum value so as to be delivered to a blood pressure determining means 30.