  A new metastable phase of the body-centered-tetragonal ruthenium ({\em
bct}--Ru) is identified to exhibit a large perpendicular magnetocrystalline
anisotropy (PMCA), whose energy, $E_{MCA}$, is as large as 150 $\mu$eV/atom,
two orders of magnitude greater than those of 3$d$ magnetic metals. Further
investigation over the range of tetragonal distortion suggests that the
appearance of the magnetism in the {\em bct}--Ru is governed by the Jahn-Teller
spit $e_g$ orbitals. Moreover, from band analysis, MCA is mainly determined by
an interplay between two $e_g$ states, $d_{x^2-y^2}$ and $d_{z^2}$ states, as a
result of level reversal associated with tetragonal distortion.
