Aluminum stabilizer, which is used in composite superconductors, in external fields at 4.2K prvides a remarkable enhancement of magnetoresistance as compared to predictions by conventional theory. In order to study the cause of this anomalous behavior, we measured the electric resistivities of copper-clad aluminum conductors having rectangular cross-sections in various magnetic fields at 4.2K. The measured values depend on the magnitude and orientation of the applied magnetic field with respect to the sample; the resistivity being several times larger for a parallel field. We calculated the current distributions in the conductors by solving Maxwell's equations, taking the Hall effect into consideration for evaluating resistivities. The predicted resistivities were consistent with our experimental results, leading us to conclude that the current caused by the Hall effect is the main origin of the anomalous magnetoresistance and its anisotropy. Finally, we discussed a method to reduce the anomalous resistivity.