We compute nuclear force in a holographic model of QCD on the basis of a
D4-D8 brane configuration in type IIA string theory. Repulsive core of nucleons
is quite important in nuclear physics, but its origin has not been
well-understood in strongly-coupled QCD. We find that string theory via
gauge/string duality deduces this repulsive core at short distance between
nucleons. Since baryons in the model are realized as solitons given by
Yang-Mills instanton configuration on flavor D8-branes, ADHM construction of
two instantons probes well the nucleon interaction at short scale, which
provides the nuclear force quantitatively. We obtain, as well as a tensor
force, a central force which is strongly repulsive as suggested in experiments
and lattice results. In particular, the nucleon-nucleon potential V(r) (as a
function of the distance) scales as 1/r^2, which is peculiar to the holographic
model. We compare our results with one-boson exchange model using the
nucleon-nucleon-meson coupling obtained in our previous paper
(arXiv:0806.3122).