To provide a different diameter fastener joint capable of connecting a plug and a socket having different diameters in a single operation.

The different diameter fastener joint 10 includes a cylindrical plug 11 having a first flange part, a first locking part and an insertion part; a cylindrical socket 18 having a second flange part and a second locking part and having an outside diameter larger than the plug; a different diameter fastener 23 exhibiting elastic force capable of fastening by a pair of locking pieces extended from an upper surface; and a fastener cover 30. In the different diameter fastener 23, the first locking piece and the second locking piece are formed with an opening therebetween. A tapered part is formed on the first flange part, the second locking piece is locked to the second locking part of the socket 18 in advance, and the first locking piece of the different diameter fastener 23 is expanded by the tapered part of the first flange part, and the plug is inserted into the socket 18, and the first locking part of the plug 11 is locked to the first locking piece of the different diameter fastener 23.