As an anode material for a Na-ion battery, we revealed the reaction mechanism of InSb for the first time: the phase separation occurred in the first sodiation to form the nanostructure in which In nanoparticles were dispersed in the Sb matrix. In the desodiation reaction, the separated phases of In and Sb did not return to the original InSb phase. The InSb electrode exhibited a good cyclability with the reversible capacities of over 400 mA h g(-1) for 250 cycles, which is better than the performances of electrodes prepared using other antimony-based alloys, Sb2Se3, FeSb2, and LaSb. The reason is that In has the favorable properties improving cyclability: (i) low thermodynamic stability of its antimonide, (ii) high electronic conductivity, (iii) low hardness, and (iv) reactivity with Na ions. On the other hand, a SnSb electrode showed a further improved performance, though the properties of Sn are very similar to those of In. This difference in the performances probably originates from the reaction mechanism: the separated phases of Sn and Sb can return to the original SnSb phase, unlike InSb. The knowledge is very noticeable for developing novel alloy-based anode materials consisting of Na-active elements.