To remarkably reduce the friction force between an outer circumferential surface of a mandrel and a fin head part of a metallic tube in expanding an end part of the metallic tube with fins fitted inside using the mandrel by providing an outer circumferential groove for lubricating oil storage formed in the circumferential direction of the outer circumferential surface of the mandrel.

This mandrel 11 for expansion having outer circumferential grooves is formed spherical or bullet-shaped. A rod 5 is integratedly fixed to its rear end. A first outer circumferential groove 12 and a second outer circumferential groove 13 are cut in the circumferential direction of the outer circumferential surface of the mandrel 11 for expansion. The size of an opening part and the depth of the outer circumferential grooves 12, 13 are appropriately determined by the viscosity of the lubricating oil. For example, when the lubricating oil low in viscosity is used, the size of the opening part is reduced, and when the lubricating oil high in viscosity is used, the size of the opening part is increased. At least one outer circumferential grooves 12, 13 is provided on the outer circumference of the mandrel 11 for expansion having the outside diameter larger than the inside diameter of the metallic tube. The mandrel 11 for expansion having outer circumferential grooves is inserted in an end part of the metallic tube having inner circumferential fins.