A sensor assembly of the present invention includes a fluid chamber. A pressure transmitting fluid disposed facing an artery passing portion of a part to be measured via a film which forms a part of an outer wall of the fluid chamber is accommodated in the fluid chamber. A pressure sensor is provided for detecting pressure of the pressure transmitting fluid as pressure applied to the artery passing portion of the part to be measured. Plate members each having a flat shape along a longitudinal direction of a wrist as the part to be measured are disposed on a side opposite to the part to be measured along the film.