A fluid bladder includes multiple segment bladders that are each formed by folding one sheet in half in a width direction X extending along an artery passing through the measurement site, welding or adhering edge portions on a side opposite to the folding location for folding in half, and welding or adhering edge portions in a lengthwise direction Y. The multiple segment bladders are stacked in a width direction orthogonal to the measurement site and integrated, and the folding locations are arranged alternatingly on opposite sides in the width direction.