The present aim is to clarify the streamwise thickness effect of a ring with rectangular cross sections, which is immersed perpendicularly to uniform flow. In a wind tunnel, we carried out measurements using a pressure transducer and a hot wire anemometer, and flow visualisations by a somke wire method, for Re=5.0×(10)3-1.5×(10)4, d/ω-4.0-8.0, and t/ω=0.29-1.30, where d. ω and t are mean diameter, cross section width and cross section thickness of the ring, respectively. Consequently, Strouhal number St, which is a reduced frequency of regular vortex shedding from the ring, is independent of t/ω, d/ω and Re. On the other hand, we can observe the maximum base suction and the maximum drag at a certain critical value of t/ω, depending on d/ω. We can clearly classify flow into two modes, viz., sub and over critical modes. Round the critical is a transition range of the modal shift. Over critical leeward extension of the after body drastically enhances the formation of vortex rings, and weakens a ring curvature effect. Only the existence of an inner sidewall is essential for the modal shift.