  Low-temperature thermal conductivity measurements were performed on single
crystal of PbTaSe$_2$, a noncentrosymmetric superconductor with topological
bulk nodal lines in the electronic band structure. It is found that the
residual linear term $\kappa_0/T$ is negligible in zero magnetic field.
Furthermore, the field dependence of $\kappa_0/T$ exhibits a clear "$S$"-shape
curve. These results suggest that PbTaSe$_2$ has multiple nodeless
superconducting gaps. Therefore, the spin-triplet state with gap nodes does not
play an important role in this noncentrosymmetric superconductor with strong
spin-orbital coupling. The fully gapped superconducting state also meets the
requirement of a topological superconductor, if PbTaSe$_2$ is indeed the case.
