Mg addition improves Tc, Birr, and Ic of the Nb3Sn formed by diffusion reaction between Nb and Ag-Sn alloy, when the diffusion reaction is performed below 650 degC. The obtained superconducting properties are similar to those of the bronze-processed Nb3Sn. The formation temperature of Nb3Sn is reduced by the Mg addition. By using OM, SEM, and field emission type EPMA, we could not measure the thickness of Nb3Sn layers, which may be thinner than 0.5 mum. It is not clear that the Mg addition increases the formation rate of Nb3Sn. However, the Nb3Sn layers should have very high Jc. The attainable overall Jc of the Nb3Sn wire are as high as those of the bronze-processed Nb3Sn wire without Ti and Ta additions, although the optimization of the cross-sectional configuration of Nb and Ag-Sn-Mg in the wire has not yet performed. Therefore the potential of the Nb3Sn conductor made by the new process seems to be very attractive for the practical applications.