  We study the mixing structure of isospin-singlet scalars, the light
quarkonium $(\bar{q}q)$ and glueball $(gg)$ in two-flavor QCD, based on a
holographic model of bottom-up hard-wall type. In the model the pure quarkonium
and glueball states are unambiguously defined in terms of the different
$U(1)_A$ charges in the restoration limit of the chiral $U(2)_L \times U(2)_R$
symmetry, in which the quarkonium gets massless as the chiral partner of the
pion. Hence the $\bar{q}q$-$gg$ mixing arises in the presence of the nonzero
chiral condensate or pion decay constant. At the realistic point where the pion
decay constant and other hadron masses reach the observed amount, we predict
the tiny mixing between the lightest quarkonia and glueball: The smallness of
the mixing is understood by the slightly small ratio of the chiral and gluon
condensate scales. The low-lying two scalar masses are calculated to be $\simeq
1.25$ GeV and $\simeq 1.77$ GeV, which are compared with masses of $f_0(1370)$
and $f_0(1710)$. Our result implies that $f_0(1710)$ predominantly consists of
glueball.
