Macroscopic and efficient spreading of excitons from an exciting laser spot to the outside area along a quasi two-dimensional space of a planar-defect in BiI 3 is presented by space and time-resolved photoluminescence, and the degenerate Four-Wave-Mixing (DFWM) signals by use of spatially separated two laser-beams. The photoluminescence shows wide spreading due to polariton-like transfer of excitons at low density and wider that at higher density. The fast transfer of excitons at high density was detected as a coherent exciton-flow by the DFWM measurements.