  The sum rule for the structure function $g_1^{p}$ which is related to the
cross section of the photo-production is used to show that a sign change of the
integral corresponding to the appropriately defined moment at $n=0$ where the
integral is cut at the point $x=x_c(Q^2)$ occurs at very small $Q^2$ near
$Q^2\sim 0.09$(GeV/c)$^2$ and $x_c\sim 0.024$. This fact shows that the origin
of the sign difference between the Ellis-Jaffe sum rule and the
Drell-Hearn-Gerasimov sum rule lies in the rapid change of the elastic
contribution at low $Q^2$ which is compensated by the inelastic contribution to
satisfy the sum rule at $n=0$. Hence it occurs at very small $Q^2$.
