A pressure vessel is provided with an annular inflation bag and an annular inflation bag for sealing the pressure vessel at intermediate positions of brachium and antebrachium of the live body in longitudinal direction of the arms of the live body, and is configured to permit a change of an internal pressure therein over a pressure range a lower limit of which is a negative value, while a portion of the brachium and antebrachium between first and second positions in the longitudinal direction is accommodated in the pressure vessel, so that the pressure vessel can be comparatively small-sized even where arterial vessel (luminal part) of a comparatively large diameter is accommodated in the pressure vessel, whereby the physical and mental burden on the subject person can be reduced.