Both high critical current densities and low hysteresis-losses are needed for Nb₃Sn strands used for poloidal coils in a nuclear fusion experimental reactor. Since our internal-tin route Nb₃Sn wires have many advantages except for hysteresis-loss, it is important to improve the loss immediately. The reason why the loss gets large with our internal-tin process is pointed out to be the bridging of filaments with one another. Hence, it is necessary to elucidate the growth mechanism of the bridging by all means in order to greatly decrease the loss. We investigated the mechanism through the cross-sectional observations and analyses of the wires at each step in an annealing pattern. According to the results, we found out that the mechanism was based on the phenomena that Cu matrix near the boundary of ε-phase bronze was lost in the pre-annealing at 450°C. As a result, the filaments in that region were approaching each other, finally leading to the occurrence of bridging. The phenomena of the disappearance of Cu matrix and the movements of the filaments were able to be explained consistently from the different diffusion rate between Cu and Sn.