  A pair of triply charmed baryons, $\Omega_{ccc}\Omega_{ccc}$, is studied as
an ideal dibaryon system by (2+1)-flavor lattice QCD with nearly physical
light-quark masses and the relativistic heavy quark action with the physical
charm quark mass. The spatial baryon-baryon correlation is related to their
scattering parameters on the basis of the HAL QCD method. The
$\Omega_{ccc}\Omega_{ccc}$ in the ${^1S_0}$ channel taking into account the
Coulomb repulsion with the charge form factor of $\Omega_{ccc}$ leads to the
scattering length $a^{\rm C}_0\simeq -19~\text{fm}$ and the effective range
$r^{\rm C}_{\mathrm{eff}}\simeq 0.45~\text{fm}$. The ratio $r^{\rm
C}_{\mathrm{eff}}/a^{\rm C}_0 \simeq -0.024$, whose magnitude is considerably
smaller than that of the dineutron ($-0.149$), indicates that
$\Omega_{ccc}\Omega_{ccc}$ is located in the unitary regime.
