Source of wedge-like dispersed sub-keV ring current ions (wedge-like structure) is investigated using Cluster CIS data. Statistics from nearly 550 traversals show that the wedge-like structure with upper energy extending to ordinary energy of ring current is found mostly in the morning sector whereas those limited within sub-keV range are found mostly in the noon-to-afternoon sector. The former is most likely the original form of the latter. With this knowledge, the Cluster traversal on 21 October 2001 turned out to be a unique observation that reveals the formation of the wedge-like structure. Spacecraft 1 and 4 detected a wedge-like structure of 0.1 ∼ 10 keV protons at 2350 UT, while spacecraft 3 did not detect it 10 min before in the same magnetic flux tube. With the observed electric field of less than 3 mV/m, this fact indicates that the dispersion started within half an hour. Pitch angle distributions of the wedge-like structure is in most cases double conic-like butterfly distributions, but the wedge-like dispersed oxygen ions during the 21 October 2001 event flow mainly from one direction (from Northern Hemisphere) without a loss cone. With its characteristic energy of 0.05 ∼ 0.3 keV, these oxygen ions originate from the Northern Hemisphere about 20 ∼ 30 min before the observation. Both the dispersion analysis and the oxygen ion tracing suggest that the observed wedge-like structure is formed in the late morning sector during the latest substorm that started at 2310 UT. Possible mechanisms of the morning source are discussed.