Abstract 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. Two numerical studies are conducted; the first imitates the RDE chamber with injection ports in a rectangular computational domain, and the second simplifies the flow field in the RDE chamber based on observations of the first simulation. The injection is parameterized by the geometrical jet and the gas conditions (i.e., premixed or non-premixed). In the first simulation of the RDE chamber, the total length of the injection nozzles constitutes 20% of the length of the chamber's lower end. The number of nozzles, N, ranges from 10 to 500 under premixed and 25 and 125 under the non-premixed condition. As the nozzle interval increases, more burned gas exists in front of the detonation wave. Under the non-premixed condition for N = 25, the propagation velocity decreases to 84% of the CJ velocity, and 82.4% of the injected C2H4 is burned by the detonation. The CJ velocity is theoretically calculated, based on the amount of burned C2H4, to be 2283 m/s, which is not close to the propagation velocity of the simulation, 1999 m/s. In the simplified model in the second study, C2H4, O2, and burned gas are quiescently placed rather than injected, and the role of the upper wall is also examined. Significant velocity decrease, down to 86% of the CJ velocity, is observed when C2H4 and O2 are placed separately with a large width of burned gas between them. The amount of incompletely combusted C2H4 is inversely proportional to the propagation velocity except in the case with an upper wall. Therefore, burned gas has a larger effect on propagation velocity in the non-premixed condition.