  The superconducting transition of the ?-pyrochlore oxide KOs2O6 with Tc =
9.60 K is studied by resistivity measurements under various magnetic fields
using a high-quality single crystal. The reentrant behavior of
superconductivity is observed near Tc in low magnetic fields below 2 T. The
recovered resistance probably due to flux flow exhibits an anomalous angle
dependence, indicating that flux pinning is enhanced in magnetic fields along
certain crystallographic directions such as [110], [001] and [112]. It is
suggested that there is an intrinsic pinning mechanism coming from the specific
crystal structure of the ?-pyrochlore oxide.
