A raw material of aluminum base alloy is extruded and formed by using a forming pattern comprising an upper pattern having plural through-holes for supplying the raw material into diaphragms of equal angles on the circumference and circular cylindrical protrusions positioned in the center of plural through-holes so as to be surrounded by plural through-holes at the exit side of the through-holes, and a lower pattern positioned in the concave portions commonly penetrating at the exit of the plural through-holes of the upper pattern, having through-holes for inserting the protrusions of circular circumference of the upper pattern by providing a tube forming gap, positioned in the center of concave portions of the concave portions in the circular columnar shape of the upper pattern.