PURPOSE: To easily and efficiently execute working for a flat tube by pressing the tube in the diametrical direction to form the flat tube after the inner part of the tube is filled with wax and the wax is solidified.

CONSTITUTION: The heated and molten wax 3 is ejected into a stainless steel tube 1, the tube is filled with the wax, the wax 3 in the tube 1 is solidified, then, the tube 1 is removed from a supporting base 2. Thereafter, the tube 1 is pressed by a press plate 5 of a press 4 in the diametrical direction to form flat tube. In this way, a flat tube having a uniform thickness one-half of the thickness of the stainless steel tube 1 is obtained in the cross section shape. Accordingly, since the wax packed in the tube is forced out when the tube is pressed and formed, the need to adjust the amt. of filled wax in the tube can be eliminated in the course of pressure forming.