Supersonic combustion in a rectangular duct was investigated experimentally. Hydrogen fuel was injected parallel to an air flow from a two-dimensional slot injector. Two-dimensional flame was established downstream of the injector and so-called precombustion shock was observed. The shock wave originated at separation region in air flow boundary layer upstream of the injector. The relationship between the strength and the position of the shock wave was different from that in perpendicular injection case. In slot injection case, it is supposed that the separation region and reacting region interacted to act as a large wedge and generate the rather strong shock. Combustion without generation of the precombustion shock was achieved with conversion duct. However, the combustion efficiency was low and the ignition delay distance was large in this case. The shock wave was expected to enhance the reaction by rising the static temperature and pressure.