The shape and size of a cavern of localized flow region around a Rushton turbine impeller, which appears in mixing a yield stress fluid under an insufficient power input, are investigated by coloring the agitated fluid with a pH indicator and by measuring velocities with LDA. An aqueous solution of carboxybinyl polymer was used as the yield stress fluid, the flow curve of which was well approximated by the Herschel-Bulkley model. A normal stress effect associated with secondary flows was observed at extremely low rotational speeds. Measurements of cavern shape and size have been carried out under the condition of Re > 10 where the normal stress effect disappears. In the case where the cavern does not meet the vessel wall, the cavern shape is like a disk with round corners and hollows near the center. After reaching the vessel wall, the cavern surface is like a disk which is round near the center. From a comparison of the contours of total velocity with those of tangential velocity, it has been confirmed that the flow is essentially tangential in the cavern. A fluid having tangential velocity below 1% of the impeller tip velocity occupies considerable volume near the cavern boundary, since the velocity variations in the region of low velocity becomes very small due to the high apparent viscosities of the fluids. It has been found that growth of the cavern based on the 1% tip velocity closely corresponds to the variation of power number with Reynolds number, and its size is well correlated with the previous model which is derived from the balance of torque acting on the cylindrical cavern surface with the impeller torque.