  We study the di-baryon in the holographic QCD. The di-baryon is composed of
six quarks and bound by the color interaction. In this paper, we adopt the
Sakai-Sugimoto (SS) model as the holographic QCD to study the di-baryon. The SS
model is formulated in a $\mathrm{D4}/\mathrm{D8}/\overline{\mathrm{D8}}$
system of the Type IIA string theory. It is expected that the di-baryon is
described by the 2-instanton configuration of the flavor symmetry on D8-branes
since the baryon number is identified as the instanton number. We will
construct the 't Hooft 2-instanton solution explicitly and use it to discuss
the stability of the di-baryon. An effective action of this model has not only
the Yang-Mills (YM) action but also the Chern-Simons (CS) action coming from
the CS action of probe D8-branes, and the CS action assigns $U(1)$ charges
associated with the baryon number to each instanton. As a result, we can see
that the di-baryon is unstable due to this $U(1)$ charge in the SS model.
