  We investigate spin accumulation caused by spin-orbit interactions (SOIs) in
a two dimensional electron system confined in one direction.
  We calculate spin density caused by three kinds of SOIs; arising from edge
potential, Rashba and Dresselhaus mechanisms. All SOIs are shown to generate
out-of-plane spin accumulations at the lateral edges of the system. We also
find that the Rashba and the Dresselhaus mechanisms are competitive.
Especially, when the strength of both interactions are equal, their effects
cancel out each other. In that case, only the edge potential mechanism becomes
relevant, and analytical expression is obtained for the spin density. The edge
potential mechanism is shown to induce spin accumulation similar to and
consistent with the experiment [Sih {\it et al}., Nature Phys {\bf 1}, 31
(2005)]. We discuss the mechanism of the spin accumulation for each SOIs in
some detail.
