  Mean field and exact diagonalization studies on the quasiparticle excitation
of an SO(5) symmetric two-leg ladder system are reported. It is shown that the
energy gap in the quasiparticle excitation is caused by the formation of rung
singlet states. We find that shadow peaks can occur above the Fermi surface
with antiferromagnetic electron correlations involving only rungs in the spin
ladder.
