An apparatus for successively estimating an intraarterial blood pressure of a living subject, based on information non-invasively obtained from a circulatory organ of the subject, including: a first device for non-invasively obtaining, from the circulatory organ of the subject, first information which relates to velocity of propagation of a pulse wave which propagates through an artery of the subject; a second device for non-invasively obtaining, from the circulatory organ of the subject, at least one of second information which relates to heart rate of the subject and third information which relates to an area defined by a volume pulse wave from a peripheral portion of the subject; and a third device for estimating, according to a predetermined relationship between (A) blood pressure, and (B1) first information and (B2) at least one of (B21) second information and (B22) third information, the intraarterial blood pressure of the subject, based on the first information obtained by the first device and the at least one of the second information and the third information obtained by the second device.