The present invention is made to accurately detect an absolute pressure even when a contact property between an elastic coupler and a body surface of an object is poor. An elastic coupler 20 is formed by an elastic material with flexibility to have two layers at least with different ultrasonic wave reflectance properties and attached to an ultrasonic wave transmitter/receiver surface, and a pressure calculating unit 30 detects the position of a boundary surface 22 between those two layers based on RF signal frame data output from an RF signal frame data selection unit 10, obtains positional change of the boundary surface based on the detected position of the boundary surface and the initial position of the boundary surface, which was obtained in advance, and obtains the absolute pressure applied to the object based on the positional change and a pre-set elasticity property of the elastic coupler. At this time, for example, an ID code is given to the elastic coupler by making the initial position or the like of the boundary surface of the elastic coupler be different in accordance with the type of the elastic coupler, the pressure calculating unit 30 identifies the ID code, identifies the type of the elastic coupler with reference to a coupler database, and reads the elasticity property corresponding to the ID code.