We have carried out positron-annihilation induced Auger-electron spectroscopy (PAES) for a basal plane surface of MoS2 as an example for a layered material. It is found that PAES is very sensitive to the impurities on the surface compared with the conventional (electron excited) Auger-electron spectroscopy. It is known that the basal plane of MoS2 is inert to oxygen. However, when the surface was exposed to O-2 gas, a strong Auger peak from oxygen appeared in the PAES spectrum. The present results suggest that positrons are trapped efficiently at surface defects such as vacancies and steps where the impurities are adsorbed preferentially.