The purpose of this study is to fabricate an anisotropic magnetorheological elastomer (MREs) based on natural rubber (NR) that has a more advanced MR effect than isotropic MREs by using an anisotropic mold. We evaluated variation in the shear modulus of the MREs in the frequency domain under a magnetic field with a continuously-variable induced current. An evaluation system is proposed that includes a magnetic flux generator, which generates a magnetic field via a continuously-variable induced current. One of the anisotropic MREs possessing 30 vol. %, 40 vol. %, and 50 vol. % carbonyl iron powder was expected to have the highest shear modulus variation as a function of the variable induced current. With the evaluation system, we identified the variation and maximum variation rate of the shear modulus of MREs with three different volume fractions of carbonyl iron powder (CIP) and continuously-variable induced current. The magnetic flux density generated by the magnetic flux generator (MFG) is optimized by electromagnetic finite element method (FEM) analysis and response surface method techniques. The values of each design factor determined by the response surface method (RSM) were applied to the redesign of the evaluation system, including the MFG. The proposed system is verified for the range in which MFG can generate magnetic flux density in order to determine the existence of magnetic saturation of the MFG by magnetic circuit analysis. Consequently, the appropriate volume fraction of CIP in MREs without inducing currents can be determined for any desired shear modulus. The desired volume fraction of CIP in the anisotropic MREs can be approximated to achieve an appropriate shear modulus variation rate. This study demonstrates that it is possible to obtain an appropriate volume fraction of CIP and induced current to achieve the desired shear modulus variation rate of an anisotropic MRE.