An apparatus for inserting flattened tubes into heat exchanger fins includes a fin stack arranging unit in which heat exchanger fins are stacked in the thickness direction and a guide is inserted into at least a cutaway portion. A flattened tube arranging unit holds flattened tubes in an intermittent arrangement with respect to the cutaway portions into which the guide has not been inserted. A flattened tube insertion driving unit inserts the flattened tubes into the cutaway portions. A contact plate produces a reactive force when the flattened tubes are inserted into the cutaway portions. A flattened tube intermittently inserted fin stack arranging unit aligns the positions of cutaway portions into which the flattened tubes have not been inserted to the positions of the flattened tubes on the flattened tube arranging unit and is used in place of the fin stack arranging unit.