Axial fuel staging is currently receiving attention to enhance the operational and fuel flexibility of gas turbines. We investigate the effects of injector spacing on combustion dynamics in an axially-staged combustor. Self-excited combustion instability measurements were performed under the identical operating conditions with three different spacings (400, 500, and 600 mm) between the primary and secondary flames, along with FEM-based linear acoustic calculations. The results show that L3-coupled instabilities mainly develop, but at a certain secondary injector location (400 mm), the instabilities were sufficiently suppressed.