PURPOSE: To obtain a highly reliable needle-shaped oxygen detector longer in the life with the formation of a strong solid electrolytic layer by melting and retreating only the solid electrolyte fastened on the surface of a metal wire applying a high energy such as plasma arc to the surface of the metal wire.

CONSTITUTION: A standard pole material such as Cr-Cr2O3 is formed on the surface of metal such as Mo with a sectional area of 0.4W9.0mm2 at a thickness of 0.1W0.3mm by spraying or the like. ZrO2(CaO), ZrO2(MgO) or the like is sprayed on the standard pole at a thickness 0.1W0.5mm to form a solid electrolytic layer. Then, a high-density energy of a tungsten/inert gas arc, a plasma arc, a lazer or the like is irradiated to re-melt the solid electrolyte alone axially moving a needle-shaped body turning to form a strong electrolytic layer. This inexpensively provides a detection with an excellent durability which is capable of detecting the concentration of free O2 in a molten steel at a high accuracy.