To keep an connected electric appliance safe and safely operate the appliance only by inserting a plug into an electric outlet by installing a terminal for earthing connected with an earth electrode, providing a function for judging the earth side and further a function as a tester and a function of adjusting the voltage.

Electric outlet entry 2 are formed in an electric outlet equipped with an extension cord provided with an electric power plug 1 and terminals 3 for earthing to be connected are formed in one side to easily earth an electric appliance. It is preferable that the electric outlet is further provided with a LED 4 to judge whether electric current flows in the electric power or not and at the same time which side is the earth side based on the lighting of the LED. Further, it is preferable that a transformer is built in the electric outlet to make supplying voltage adjustable by operating a tap switching switch 5 while monitoring a voltmeter 6. At that time, the transformer is so installed as to work as countermeasure against a lightning surge. Moreover, a buzzer 7 giving an alarm when overcurrent being detected is preferably arranged to prevent damage to an appliance connected with the electric outlet.