The authors experimentally studied a shear flow of a water-based magnetic fluid between concentric annuli. The experiment was carried out by changing the roll accerelation of the outer cylinder, and the moment acting on the inner cylinder was measured. Experimental results were discussed in the following three ways. At first the magnetic fluid was treated as a Newtonian fluid, and a small difference in the flow curves was found. Next the power law theory was applied in order to explain these differences. Finally the authors examined this flow problem using the micropolar theory, which gives the theoretical relation between the moment acting on the inner cylinder and the roll accerelation of the outer one. In this relation, a coefficient denotes the micropolar effect. The authors measured this value and showed the effect of the substructure on the fluid.