The internal flow and wave structure of a nozzle-less, cylindrical rotating detonation engine (RDE) was investigated in this research. For a C2H4–O2 mixture, pressure distributions and chemiluminescence inside the chamber were obtained by combustion experiments. Pressure distributions suggest that combustion region is finished near 20 mm from bottom of the cylindrical RDE, and Mach number distributions obtained by Rayleigh flow theory also reveal flow reaches the sonic speed at the exit of the cylindrical RDE. Chemiluminescence images taken from the side of the cylindrical RDE show that strong luminance area ends at approximately 15-20 mm, which also means that combustion in the RDE finishes around that point. Moreover, a forward-tilting detonation wave which stably rotated at 1414 m/s was observed in the images, and it extended to the downstream of burned gas. From these results of strong luminescence at bottom and shock wave extending to the exit, the flow and wave structure inside cylindrical RDEs are proposed.