A prototype system for non-invasively measure instantaneous blood pressure (BP) in radial artery was newly devised. The system was based on the volume-compensation method, and several components were newly designed, i.e., a disk-type cuff for local pressurization and a nozzle-flapper type electro-pneumatic converter (EPC) for the cuff-pressure control. Results of experiments indicated that, using the disk-type cuff, the radial artery could be completely compressed, and newly devised EPC had sufficient frequency response for BP measurement in human. Also indicated was that the prototype system consisted with these component was capable of non-invasively measuring instantaneous blood pressure in radial artery, not only in rest condition, but also in a stressful condition such as cold pressor test.