When a supersonic flow is decelerated to subsonic flow in a duct, a series of plane or lambda shocks, known as a pseudo-shock wave, is produced as the result of shock wave-turbulent boundary layer interaction. This paper describes an experimental study on the locations and shapes of pseudo-shock waves in straight rectangular ducts when the flows are choked at the duct exits. The experiment was done for Mach number at the duct entrance M∞ from 1.72 to 1.88 and for duct length L/D = 13.8~23.6, where L is the length of the duct and D (= 50mm) the width of the duct. It is shown that the locations of pseudo-shock waves move in a downstream direction with decreasing L/D and increasing M∞. As the locations of pseudo-shock waves move in a down-stream direction, the shapes of pseudo-shock waves change from λ -type to X-type. It is also shown that the location of the pseudo-shock wave in a duct can be predicted by the theory of the Fanno flow with normal shock wave when the flow is choked at the duct exit.