A two-dimensional investigation is conducted to clarify the velocity decrease mechanism of a detonation wave propagating within a Rotating Detonation Engine (RDE) chamber, focusing on the effect of injection conditions of the fuel and oxidizer. RDE chamber with injection ports is imitated in a rectangular computational domain, and the key factor for the velocity decrease is traced by changing the geometrical jet conditions. Although more burned gas exists in front of the detonation wave as the nozzle interval increases, it does not affect the propagation velocity under the premixed C2H4 + 3O2 gas injection. The velocity decrease is observed under the conditions that C2H4 and O2 are separately injected; adjacent and non-adjacent gas injections. The lowtemperature area and the incomplete combustion of C2H4 are generated behind the detonation wave, and the insufficient mixing with fuel and oxidizer would be one of the key factors for the velocity decrease. However, CJ velocity theoretically calculated from the mass of burned C2H4 becomes larger than the propagation velocity. Then, the burned gas in front of the detonation wave is also investigated through the cases for the non-premixed gas injection. The velocity decrease and the amount of burned gas in front of the detonation wave are restrained when the nozzle interval becomes smaller, so the effect of burned gas in front of the detonation wave on the velocity decrease for the nonpremixed gas injection is confirmed. This relationship is not observed in the comparison of the adjacent and the non-adjacent injections, and further study is needed to clarify the difference between the mechanisms for velocity decrease in these two injection conditions.