Disclosed is a method for welding tape cartridge reels, with ensuring a high level of parallelism between both their flange portions. Specifically, this method for welding a tape cartridge reel, the tape cartridge reel including a flange having a welding rib and a flange-integrated reel hub, the method includes: pressing the flange against the flange-integrated reel hub by using an ultrasonic horn, while vibrating the flange by means of an ultrasonic wave from the ultrasonic horn, so that the flange is welded to the flange-integrated reel hub through the welding rib; and terminating a welding operation of the ultrasonic horn, while the flange is in contact with the flange-integrated reel hub only through the welding rib.