This paper reports an experimental study on circular duct flow superposed with swirling flow. In this study, three time-mean velocity components of swirling flow in circular duct is measured using five-hole pitot probe when the swirling numbers are changing form 0 to 1. 35 by the axial flow type swirler. Further, the flow visualization is carried out using a streamer and the spark tracing method, and the flow patterns become clearer. From these results, the present study clarifies the location, size and shape of recirculation, eye of vortex and sub-recirculation zones for swirling numbers, and maximum reverse flow rate of a recirculation zone.