PURPOSE: To provide a joint surface of cylindrical tubes finished with high accurracy in view of a processing characteristic by drilling and grinding the cylindrical tubes along their joint surface by means of a rotary tool with a conical surface.

CONSTITUTION: The position of a point V is previously defined. Taking the point as a fixed point, both cylindrical tubes 10, 11 are cut or ground such that a rotary tool makes contact with a mutual cross line 13 formed by internal peripheral surfaces of the cylindrical tubes 10, 11. Thereafter, the position of a point U is specified in tube 10 but outside the point V. Taking point U as a fixed point, the cylindrical tubes are further cut and ground such that the rotary touches the cross line 13 formed by cylindrical surfaces formed between external and internal peripheral surfaces of both cylindrical surfaces 10, 11. Hereby, a beveling 16 is processed to provide a precision joint surface.