In order to examine experimentally the growth behavior of Nb3Sn during reactive diffusion between Nb and a bronze with the alpha beta two-phase microstructure, a sandwich (Cu-Sn-Ti)/Nb/(Cu-Sn-Ti) diffusion couple was prepared from pure Nb and a ternary Cu-Sn-Ti alloy with concentrations of 9.3 at.% Sn and 0.3 at.% Ti by a diffusion bonding technique. Here, alpha is the primary solid-solution phase of Cu with the face-centered cubic structure, and beta is the intermediate phase with the body-centered cubic structure. The diffusion couple was isothermally annealed at temperatures between T = 923 and 1,053 K for various times up to 843 h. Owing to annealing, the Nb3Sn layer is formed along each (Cu-Sn-Ti)/Nb interface in the diffusion couple, and grows mainly into Nb. Hence, the migration of the Nb3Sn/Nb interface governs the growth of the Nb3Sn layer. The mean thickness of the Nb3Sn layer is proportional to a power function of the annealing time. The exponent of the power function is close to unity at T = 923 K, but takes values of 0.8-0.7 at T = 973-1,053 K. Consequently, the interface reaction at the migrating Nb3Sn/Nb interface is the rate-controlling process for the growth of the Nb3Sn layer at T = 923 K, and the interdiffusion across the Nb3Sn layer as well as the interface reaction contributes to the rate-controlling process at T = 973-1,053 K. Except the effect of Ti, the growth rate of the Nb3Sn layer is predominantly determined by the activity of Sn in the bronze and thus the concentration of Sn in the alpha phase. As a result, the growth rate is hardly affected by the volume fraction of the beta phase, though the final amount of the Nb3Sn layer may depend on the volume fraction.