  The mechanism of superconductivity in ${\rm Sr}_{2}{\rm RuO}_{4}$ is studied
using a degenerate Hubbard model within the weak coupling theory. When the
system approaches the orbital instability which is realized due to increasing
the on-site Coulomb interaction between the electrons in the different
orbitals, it is shown that the triplet superconductivity appears. This
superconducting mechanism is only available in orbitally degenerate systems
with multiple Fermi surfaces.
