To easily connect an existing article or the like with another member.

This connection device 1 includes a first joining structure 13 capable of being joined with an existing article and a second joining structures 11, 12 capable of being joined with another member by a joining method different from that of the first joining structure. The structures 11, 12 are provided with a guide mechanism and a fixing mechanism. The guide mechanism is provided on the outer peripheral face of either the device 1 or the member and on the inner peripheral face of the other for rotating one of the two members relative to the other when one is pressed toward the other along an axis. The fitting mechanism has a first press fitting part formed on the outer peripheral face of one member and a second press fitting part formed on the inner peripheral face of the other member. When one of the two members is rotated relative to the other along the guide mechanism, either the first press fitting part or the second press fitting part is rotated relative to the other and either the first press fitting part or the second press fitting part is press-fitted into the other for the two members to be mutually fitted and fixed.