The purpose of this study is to clarify the internal flow pattern in several curved diffusers with the different logarithmic spiral angle. The flow visualizations were carried out by smoke wire method using High Speed Video Camera, and their results were compared with the data obtained by the wind tunnel experiment. In this study, the effects of the existence of the rear duct connected at the outlet of the curved diffuser on the flow patterns were clarified by the flow visualization.As a result of this study, it was found that the performance of the curved diffuser was improved considerably by the existence of a guide vane and a rear duct of suitable length, and was affected by the difference of logarithmic spiral angle, and the flow patterns obtained by the flow visualization agreed comparatively with the wind tunnel experiment.