Metal pipe and manufacturing method thereof

Metal tube according to the invention has a plurality of different heights, it extends axially provided on the inner peripheral surface a plurality of convex portions arranged in a circumferential direction. Even if the scale of the streak structure or hairpin vortices by the Reynolds number varies changed, the streak or hairpin vortices meet one of the protrusions. Accordingly, it is possible to reduce the fluid friction over a wide Reynolds number range.