  We investigate a systematic error coming from higher excited state
contributions in the energy shift of light nucleus in the two-nucleon channel
by comparing two different source calculations with the exponential and wall
sources. Since it is hard to obtain a clear signal of the wall source
correlation function in a plateau region, we employ a large quark mass as the
pion mass is 0.8 GeV in quenched QCD. We discuss the systematic error in the
spin-triplet channel of the two-nucleon system, and the volume dependence of
the energy shift.
