  Energy spectrum of doubly heavy tetraquarks, $T_{QQ}$ ($QQ \bar{q} \bar{q}$
with $Q = c, b$ and $q = u, d, s$), is studied in the potential chiral-diquark
model. Using the chiral effective theory of diquarks and the
quark-diquark-based potential model, the $T_{bb}$, $T_{cc}$, and $T_{cb}$
tetraquarks are described as a three-body system composed of two heavy quarks
and an antidiquark. We find several $T_{bb}$ bound states, while no $T_{cc}$
and $T_{cb}$ (deep) bound state is seen. We also study the change of the
$T_{QQ}$ tetraquark masses under restoration of chiral symmetry.
