  We investigate the superconducting gap function of topological superconductor
PbTaSe$_2$. Temperature, magnetic field, and three-dimensional (3D) field-angle
dependences of the specific heat prove that the superconductivity of PbTaSe$_2$
is fully-gapped, with two isotropic $s$-wave gaps. The pair-breaking effect is
probed by systematically increasing non-magnetic disorders through
H$^+$-irradiations. The superconducting transition temperature, $T_{\rm{c}}$,
is found to be robust against disorders, which suggests that the pairing should
be sign-preserved rather than sign-reversed.
