To provide a connection failure detecting device of a plug-in type terminal part, capable of detecting connection failure at an early stage without changing the design of the terminal part connected by inserting a conductive bar and a plug in it.

In this plug-in type terminal part connected by inserting the conductive bar 1 in it, a blade spring 4 abutting on the tip of the inserted conductive bar 1, and a separator 5 disposed between the blade spring 4 and a terminal metal fitting 3 so that both are not brought into contact with each other are provided, and a connection failure detecting circuit 6 to detect connection failure between the conductive bar 1 and the terminal metal fitting 3 by a potential difference generated between the terminal metal fitting 3 and the blade spring 4 is provided. The connection failure detecting circuit 6 is provided with a resistance element 8 disposed between the blade spring 4 and the terminal metal fitting 3, and is equipped with a photo coupler switched on by the voltage drop of the resistance element 8.