In this study, the change characteristics of the flow structure due to the circularization of the right-angled corner in a straight open channel were investigated using flow velocity measurements and flow visualization. It was clarified that when the water depth is low, where all the corners are curved, the scale and strength of the elliptical swirling flow formed on the water surface side increases compared to the right-angled corner. It was also found that the vertical and horizontal Reynolds stress distributions of the circular corner were significantly different from the characteristics of the right-angled corner. When the water depth increased, the effect of the circular corner was limited to the vicinity of the curved surface, and it was inferred that the flow field was similar to the right-angled corner. Furthermore, it was clarified that the longitudinal vortex structure near the water surface at the circular corner has a unique feature that it is larger than that at the right-angled corner.