An AC adaptor that can reduce power not in use state, and a battery-operated electronic apparatus that can employ such an AC adaptor. An AC adaptor includes, (a) an input line along which an external AC voltage is input, (b) a first transformer for transforming the input AC voltage into a DC voltage at a first voltage level, (c) a switch provided on the primary side of the first transformer, (d) an output line along which a DC voltage on the secondary side of the first transformer is output externally, (e) a second transformer for transforming the input AC voltage into a DC voltage at a second voltage level that is higher than the first voltage level and for outputting the resultant DC voltage to the output line, and (f) an AC adaptor operation control circuit for comparing a voltage level across the output line with a reference voltage level, which is higher than the first voltage level and which is lower than the second voltage level, and for turning on the switch when the voltage level across the output line is lower than the reference voltage level, or for turning off the switch when the voltage level across the output line exceeds the reference voltage level. When an electronic apparatus does not require supply of power, the AC adaptor is detached from the system load in the apparatus, so that the voltage across the output line of the AC adaptor exceeds the reference voltage. In response to the result of the comparison, the switch on the primary side of the first transformer is turned off, and thus the operation of the first transformer can be halted.