  Recently it is claimed that there is a significant systematic error from
excited state contributions in the nucleus correlation functions by comparing
with calculations using the exponential and wall source operators. However, the
wall source result is obtained in much earlier time than the plateau region. In
order to investigate the systematic error in the plateau region, we calculate
the correlation functions with both the operators in quenched QCD at 0.8 GeV
pion mass and in $N_f=2+1$ QCD at 0.7 GeV pion mass in high accuracy. In this
report we present preliminary results of those calculations, and show that the
energy shift obtained from the two sources agree with each other, if those are
determined from a region, where both the nucleon and two-nucleon correlation
functions have plateaus.
