When a surface of a titanium disk was melted in an atmosphere of pure nitrogen using a 3D Micro Welder which was designed by the present authors, the surface was nitrided to a depth of 90 to 260 mu m depending on the arc current of 6 to 24 A. The concentration of nitrogen in the nitrided layer was approximately 50 mol% at the surface, and the concentration decreased as the distance from the surface increased. A TiN layer was formed at the surface, and beneath the TiN layer, a dual phase layer of TiN and alpha-Ti was formed. Vickers hardness was approximately 1800 in the TiN layer and it varied from 900 to 200 in the dual phase layer as the distance from the surface increased.