PURPOSE: To determine a blood pressure value without utilizing a korotokoff's sound at all and to exactly measure blood pressure without being influenced with a noise by arranging a detection quantity on the center side from a blood vacant part, and detecting a pulse wave going to the blood vacant part as a pressure fluctuation.

CONSTITUTION: A CPU can control freely pressure of a blood vacant quantity 210 and a detection quantity 220 by giving an instruction to an air pump 171 and a stepping motor 173. When a leakage valve 172 is fully closed by driving the stepping motor 173, and air is fed into conduit tubes 230, 240 by operating the air pump 171, pressure in a cavity can be increased. When a binding band 200 is installed in the upper arm part of a living body, a pressure fluctuation is generated at every pulsation period, and this pressure fluctuation corresponds to a pulse wave in the upper arm part. A blood vacant cavity 210 is a larger cavity than a detection cavity 220, and the capacity of air becomes considerably large. Accordingly, a pressure fluctuation by a pulse wave is not detected enough by the blood vacant cavity 210, but can be detected enough by the detection cavity 220. In this regard, the detection cavity 220 is positioned in the center side of the blood vacant cavity 210.