NOVELTY - A tubular shaped steel plate (1) is molded and butt end surface (4) of the plate is welded. Plasma working gas sprays anode gas on the plasma gas generated by applying a voltage between cathode and anode, and subjected to plasma injection as plasma working gas in cathode gas. The laminar-flow plasma (5) or pseudo laminar-flow plasma is sprayed with respect to at least butt end surface of the portion which is at 650 degrees C or more in a welding upstream than in the welding point of the electro-resistance welding. Thus, the steel conduit tube is manufactured.    USE - Manufacture of steel conduit tube for oil well pipe, line pipe for petroleum or natural gas, steel pipe for nuclear power, geothermal, chemical processing plant and machine structure, and general piping.    ADVANTAGE - The method efficiently provides steel conduit tube reduces the welding defects from oxides by stable irradiation of plasma.    DETAILED DESCRIPTION - A tubular shaped steel plate is molded and the butt end surface of the plate is welded using electroresistance welding. Plasma working gas from the cascade type plasma gun (20) sprays anode gas on the plasma gas generated by applying a voltage between cathode and anode, and subjected to plasma injection as plasma working gas in cathode gas. The plasma working gas contains hydrogen gas (2-50 volume%), remainder is argon gas and unavoidable contaminant gas or remainder is mixed gas and unavoidable contaminant gas in which the mixed gas contains nitrogen, helium and/or argon gas. The reducing high temperature laminar-flow plasma or reducing high temperature pseudo laminar-flow plasma which has reduction property is sprayed with respect to at least butt end surface of the portion at 650 degrees C or more in a welding upstream than in the welding point of the electro-resistance welding. Thus, the steel conduit tube is manufactured. The plasma working gas contains methane and ethane gas. The steel sheet contains silicon or chromium.    DESCRIPTION OF DRAWING(S) - The drawings show the schematic view of the manufacturing method of steel conduit tube.    Steel plate (1)    High frequency coil (2)    Butt end surface (4)    Laminar-flow plasma (5)    Plasma gun (20)