PURPOSE: To make a fin difficult to bend and to increase an area by a method wherein a number of holes through which a pipe is inserted and a number of lines of protrusion formed between the holes and in parallel to each other are formed in each of fins arranged with distance therebetween.

CONSTITUTION: A title heat exchanger is formed with a number of plateform fins 2 made of a metal aligned with a distance therebetween, a pair of pipe plates 3 and 3 made of a metal on both sides, and a number of pipes 4 made of a metal extending through the fins 2 and the pipe plates 3 and 3 at right angles. The fin 2 is formed of thin aluminum in a square, and a number of holes 5 aligned obliquely at equal intervals and a cylinder part protruded in one-way from the peripheral edge of the hole is formed, and lines 7 of slender protrusion are formed in parallel to each other between the holes 5 and protrud ed in a reverse direction to the cylinder part 6. This constitution causes the line 7 of protrusion formed on each fin 2 to work as a reinforcing line and enables the increase of the area of the fin 2.