A plug-in unit includes a breaker electrically connected with a busbar at a power supply side and an electric device electrically connected with the breaker. Joint surfaces making contact with each other are formed at the breaker and the electric device, respectively. The breaker includes a terminal metal fitting connecting the busbar thereto and a plug-in terminal metal fitting installed at the joint surface of the breaker, and the electric device includes a plug terminal, which is installed at the joint surface thereof and capable of being connected with the plug-in terminal metal fitting. The breaker is detachably joined with the electric device by an engaging member including an engaging unit and an engagement receiving unit. The engaging unit and the engagement receiving unit are installed at the joint surfaces of the breaker and the electric device, respectively, and detachably engaged with each other.