  The thermal conductivity of the doped topological crystalline insulator,
Sn$_{0.6}$In$_{0.4}$Te superconducting single crystal with $T_c =$ 4.1 K, was
measured down to 50 mK. It is found that the residual linear term $\kappa_0/T$
is negligible in zero magnetic field. The $\kappa_0/T$ shows a slow field
dependence at low magnetic field. These results suggest that the
superconducting gap is nodeless, unless there exist point nodes with directions
perpendicular to the heat current. Due to its high-symmetry fcc crystal
structure of Sn$_{0.6}$In$_{0.4}$Te, however, such point nodes can be excluded.
Therefore we demonstrate that this topological superconductor candidate has a
full superconducting gap in the bulk. It is likely the unconventional
odd-parity $A_{1u}$ state which supports a surface Andreev bound state.
