  A quantum dot with spin-orbit interaction can work as an efficient spin
filter if it is connected to N (> 2) external leads via tunnel barriers. When
an unpolarized current is injected to a quantum dot from a lead, polarized
currents are ejected to other leads. A two-level quantum dot is examined as a
minimal model. First, we show that the spin polarization is markedly enhanced
by resonant tunneling when the level spacing in the dot is smaller than the
level broadening. Next, we examine the many-body resonance induced by the Kondo
effect in the Coulomb blockade regime. A large spin current is generated in the
presence of the SU(4) Kondo effect when the level spacing is less than the
Kondo temperature.
