This study examined the roles of heat fluxes from the Kuroshio Current in enhancing a frontal convective rainband associated with an extratropical cyclone that brought record-breaking heavy rainfall to Miyake Island, Japan, in January 2017. A simulation of the rainband (control experiment) and sensitivity experiments without the sensible and latent heat fluxes from the Kuroshio Current were conducted using a regional cloud-resolving model. The rainband developed along a non-classic front (outer front), which formed to the north of a warm front associated with the cyclone. The control experiment reproduced the intensity and migration of the rainband well. As the rainband developed, the heat fluxes from the Kuroshio Current became evident around the cold conveyor belt of the cyclone to the south of the rainband. The latent heat fluxes were similar to 2.3 times larger than the sensible heat fluxes. Comparisons between the control and sensitivity experiments demonstrated that the heat fluxes, especially the latent heat fluxes, enhanced the rainband. The sensible heat fluxes slightly intensified convective instability in the lower troposphere, whereas the latent heat fluxes significantly increased the near-surface moisture content and the convective instability. A frontal updraft along the outer front released the increased convective instability, which intensified the moisture convergence, condensation, and updraft, enhancing the rainband. The findings show that the heat fluxes from the Kuroshio Current, especially the latent heat fluxes, enhanced the heavy rainfall-producing rainband by increasing the moisture content and the convective instability.