There are two types of stroma in interlobular stromal fibrous tissue: (1) a surrounding stroma that is densely packed with fibrous connective tissues and surrounds the lobules and an extralobular duct (isoechoic) and (2) an edematous stroma with loosely packed fibrous connective tissues (hyperechoic). The isoechoic structure includes “extralobular duct - TDLU” in the surrounding stroma. The isoechoic structure shows the running of the duct. Normal isoechoic structures have two types of regularities, i.e., toward the nipple and toward the boundary surface between the lobes, reflecting the course of the mammary ducts. The continuity of the isoechoic structure should be traced by paying attention to these two types of regularities. The site where the isoechoic structure is interrupted in its continuity or disordered in direction should be evaluated for possible pathological changes.