To decrease the frictional resistance during the drawing process of a long high alloy tube of small diameter in the high-pressure lubricant oil, and prevent the infinitesimal fluctuation caused by the sound phenomena in the inner diameter of the tube in the axial direction. Afore said result is obtained by forming an aluminum oxide coat on the surface of the cemented carbide plug via a coat of single or mixed layers of titanium carbide, nitride, or carbide nitride.

To form coat 3 of Al2O3 on the peripheral surface of a cylindrical shaped plug body via titanium compound coat 2 consisting of a single or mixed layers of TiC, TiN or TiCN. Both of the titanium compound coat 2 and the aluminum oxide coat 3 are formed by the common method of physical or chemical evaporation, but it is desirable to use the method of chemical evaporation for a coating of higher accuracy. The thickness of the coating is not limited but the thickness of the titanium compound coat 2 is desirable to be around 1 to 5 mm, and the aluminum oxide coat 3 is around 2 to 4 mm.