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The paper proposes a new approach for the fabrication of MgB2 tape utilizing an interface diffusion reaction between boron (B) and Fe-Mg alloy which has workability to be deformed into a tape form. The composite of an Fe-Mg alloy substrate tape with B layer on its surface was prepared by a dip coating process. During the subsequent heat treatment at 700deg C, Mg in the Fe-Mg alloy substrate diffuses into the coated B layers and reacts with B to form MgB2 superconducting layer. The reaction proceeds from the Fe-Mg/B interface to the inside of B layer and such a formation mechanism produces a dense structure of MgB2 with much less MgO impurity, which is difficult to achieve by the conventional PIT process. SiC addition to the B layer is effective at improving the critical current density Jc in applied magnetic fields as in the case of the PIT process. A transport Jc value of 2,000 A/cm2 has been obtained at 4.2 K and 4 T for the Fe - 20 at%Mg/(B + SiC) composite tape. This approach is expected to be a breakthrough in the development of MgB2 wire with a high quality superconducting layer in the composite.