  We consider the possibility of the lightest sterile neutrino dark matter
which has dipole interaction with heavier sterile neutrinos. The lifetime can
be long enough to be a dark matter candidate without violating other
constraints and the correct amount of relic abundance can be produced in the
early Universe. We find that a sterile neutrino with the mass of around MeV and
the dimension-five non-renormalisable dipole interaction suppressed by
$\Lambda_5 \gtrsim 10^{15}$ GeV can be a good candidate of dark matter, while
heavier sterile neutrinos with masses of the order of GeV can explain the
active neutrino oscillations.
