Recently, additive manufacturing has been receiving considerable attention as a cost-effective technology compared to traditional manufacturing methods for its capability of fabricating complex parts directly from feedstock materials. In order to investigate possibilities of wire-arc additive manufacturing of commercially pure titanium by metal inert gas welding under atmospheric condition, fundamental experiments mainly focusing on contamination during welding process and its influence on mechanical properties of welded metal were conducted. From such experimental results, it was clarified that nitridation attributes largely to hardening and embrittlement of welded metal. Moreover, it was shown that promoting retention of shielding gas from welding torch or after shielding nozzle may be an effective solution to lower such risk of contamination or embrittlement problem.