A compact vehicle-mounted electrical outlet box offering a reliable plug blade detection function. The electrical outlet box is formed from synthetic resin rectangular case 1 which includes body 2 and cover 3. A pair of electrical plug blades 100 and 101 are inserted into plug blade insertion slots 5 located on the front surface of case 1 through which the plug blades contact two blade receiver parts 10. The forceful contact of plug blades 100 and 101 rotate two pivot levers 12 which convert the insertion force of plug blades 100 and 101 to a directional force. Two return springs 13 apply pressure to pivot levers 12 in a direction opposite to that applied by the insertion of plug blades 100 and 101. Printed circuit block 14, which is comprised of circuit base plate 14a to which each microswitch 14b is installed, is housed within case 1. The two pivot levers 12 are located at the approximate mid-point of body 2 between the two mutually facing blade receiver parts.