PURPOSE: To increase the thickness to the inner peripheral surface side of a metallic tube of longitudinal axis.

CONSTITUTION: While heating a part of a metallic tube 11 by a heating means 21 from the peripheral direction, the outer peripheral surface of the heated part 11a of a metallic tube 11 is cooled forcedly by a cooling means 31 and while keeping the temperature on the outer peripheral surface side of the heated part 11a of the metallic tube 11 below the temperature on the inner peripheral surface side, the metallic tube 11 is compressed by a compression means 15 in the axial direction, the inner peripheral surface of the heated part 11a is bulged out inward in the radial direction.