PURPOSE: To easily and evenly draw and expand a tube even when a tube is such that the thickness is uneven, a welded part exists or the tube is a hard kind.

CONSTITUTION: In a method for expanding a tube by force-fitting a core bar shaped jig which is provided with an outside diameter equivalent to an inside diameter to be expanded, a jig 1 is used on which a plural number of projected lines 5, 5,... are provided at mutually specific spaces apart in the shaft direction on the outer circumferential surface. Each projected line 5 is force-fitted along the inner circumferential surface of a tube 2 by force-fitting the jig 1 from the end of the tube 2; and, limiting in between the projected lines 5 the area in the circumferential direction of the tube, the material is each drawn and expanded within the section in between the projected lines.