To provide an easy-to-install plug-in fitting that allows a user to visually check an engagement part of the plug-in fitting thoroughly.

A plug-in fitting 1 is electrically connected to a bus duct body 10 by engagement. The plug-in fitting 1 includes an outer box 2. The outer box 2 includes a bar 20 as a connecting member which is arranged on a bottom surface 8 of the outer box 2 to be engaged with the bus duct body 10 side, an inspection window 5 which is formed at a position corresponding to the bar 20, and a transparent insulator 6 arranged on the bottom surface 8 of the outer box 2 corresponding to the bar 20.