PURPOSE: To ensure hold of a bobbin near the terminal end of a bobbin holder by providing an extension member on a cap, and bringing this extension member into contact with a spacer ring or an elastic ring on the way of extension.

CONSTITUTION: The extension member (extension cylinder) 3a of a cap 3 makes contact with spacer rings 4a, 4b or elastic rings 5c, 5d on the way of extension. Thus, the transfer point of compressing force is divided into the start end and the middle, and a sufficient compressing force is delivered to the elastic rings 5c, 5d from the middle to the terminal end.