  We use the Feynman--Kleinert variational method [R.P. Feynman and H.
Kleinert, Phys. Rev. A {\bf 34}, 5080, (1986)] to calculate the partition
function and effective pinning energy of a single flux line near a columnar pin
in type II superconductors. It is found that there is a phase transition
between a low temperature phase where the flux line is localized near the
columnar pin and where its internal modes fluctuations are bounded, and a high
temperature, depinned phase where the flux line is delocalized and its internal
fluctuations are those of a free line.
