PROBLEM TO BE SOLVED: To provide an AC adaptor which is so arranged that AC power from a commercial power source is not consumed.
SOLUTION: The AC from the commercial power source is supplied from an AC plug 1 to a primary winding of the transformer 21 of the AC adaptor 2. A rectifying circuit 22 and a smoothing circuit 23 are connected to a secondary winding of the transformer 21, and a DC being outputs of these circuits is sent to a DC plug 3. One terminal 21b of the primary winding of the transformer 21 is connected to one terminal of the AC plug 1 via a normally-open switch provided in the DC plug 3 by a cable 12. The switch is closed by an operation pin 34 pressed by a casing or the like when the DC plug 3 is connected to an electronic apparatus such as a personal computer.