  We report low-temperature heat transport measurements on superconducting iron
sulfide FeS with $T_c \approx$ 5 K, which has the same crystal structure and
similar electronic band structure to the superconducting iron selenide FeSe. In
zero magnetic field, a significant residual linear term $\kappa_0/T$ is
observed. At low field, $\kappa_0/T$ increases rapidly with the increase of
field. These results provide strong evidence for nodal superconducting gap in
FeS. We compare it with the sister compound FeSe, and other iron-based
superconductors with nodal gap.
