An electronic device, such as a personal computer, has a rectangular main body including a bottom wall and a side wall which has an opening. A circuit board is arranged in the main body to oppose the bottom wall within a predetermined distance. The circuit board and the bottom wall define a mounting space therebetween. An expansion board is mounted in the mounting space and detachable through the bottom wall and the side wall. A connector is fixed to and electrically connected to the circuit board and located between the circuit board and the bottom wall. The connector has a connecting portion opposing the opening and allowing connection of the expansion board thereto. The connector also supports the circuit board. A restraining system is provided for restraining movement of the connector with respect to the bottom wall.