  A theoretical proposal for testing Bell's inequality in mesoscopic systems is
presented. We show that the entanglement of two electron spins can be detected
in the spin filter effect in the mesoscopic semiconductor / ferromagnetic
semiconductor / semiconductor junction. The current-current correlation
function is calculated by use of the quantum scattering theory and we compare
it with the local hidden variable theory. We also discuss the influence of an
imperfect spin filter and derive the condition to see the violation of Bell's
inequality experimentally.
