Flows around a circular cylinder in oscillatory flow were studied by a numerical simulation using a finite-volume method. The computations were carried out by assuming 3-dimensional (3-d), unsteady, incompressible and viscous flow through Keulegan-Carpenter number (KC) in the range 3≤KC≤25 at Stokes number (β) of 95. The predictions obtained by 3-d computations were compared with experimental results by using a U-tube water tank. As a result, typical flow-patterns induced by oscillatory flow, for example, longitudinal vortices at KC=3∼4, around KC=12, pattern around KC=16 and vortex at KC≥20 were able to be successfully reproduced. The computed flow-patterns were in good agreement with experimental results. The collapse of 3-d vortical cell-structure at KC=5 causes a temporal change of the direction of vortex-shedding. Furthermore, it is found that the flow-patterns of transverse-vortex street, double-pair and wake vortex street have the 3-d vortical structures which changes complicatedly in the span-direction.