To efficiently conduct assembling of a drum spring in an antenna holder that is a component for fixing a small sized antenna used for a portable telephone set.

The insertion device consists of a set of die base 8 that is divided into a base 6 and a cover 7 with a hinge 5 and a press-fit rod 10. The base 6 and the cover 7 are engraved to contain an antenna holder 2 and a drum spring 4 placed in series. After the antenna holder 2 and the drum spring 4 are mounted on the base 6, the cover 7 is closed. When the diameter of the drum spring 4 becomes somewhat smaller than the inner diameter of an open tip of the antenna holder 2, the press-fit rod 10 is advanced so as to press-fit the drum spring 4 into the antenna holder 2.