  We have performed a systematic study of the ground state for even-even 1,002
nuclei in which 28 octupole-deformed nuclei are found. The interplay between
the spatial deformation and the pairing correlation, plays important roles in
nuclear structure. Our model is based on the Skyrme Hartree-Fock plus BCS model
represented in the three-dimensional Cartesian coordinate space which can
describe any kind of nuclear shape. The quadrupole and octupole deformed nuclei
appear in the mass region with characteristic neutron and proton numbers which
are consistent with previous studies. In our results, there appear only pear
shape ($\beta_{30}$) in the octupole deformed nuclei. We investigate the
potential energy surfaces as functions of the octupole deformations
($\beta_{3m}, m = 0, 1, 2, 3$), which tells us that. $^{220}$Rn has also local
minima in the $\beta_{31}$ and $\beta_{32}$ potential energy surfaces.
