A fluid bladder has a first fluid bladder region that inflates due to receiving a supply of fluid and is contained in a portion of an outer circumferential surface of a measurement site in a lengthwise direction of a cuff, the portion corresponding to a first half surface, where an artery is. Also, the fluid bladder has an expansion region that is expanded due to an action from outside, and is contained at a portion of the outer circumferential surface of the measurement site that corresponds to a second half surface, in the lengthwise direction of the cuff. During inflation for blood pressure measurement, a stroke amount by which the expansion region expands is larger in the thickness direction than a stroke amount by which the first fluid bladder region swells.