An AC adaptor has a switching regulator for converting alternate current to direct current. Filters are provided to filter conducted noise, and a filter switcher switches to the filter to be used in the AC adaptor. A detector detects power consumption state of an electrical apparatus. A state-filter relations storage stores correlations between the power consumption states of the apparatus and the filters to determine the filter that meets requirements so that the conducted noise to be filtered in the AC adaptor is suppressed to a standard limit value or less and so that power consumption is made lowest among the filters. A filter switching controller controls the filter switcher to switch to the filter to be used in the AC adaptor in correlation to the detected power consumption state of the electrical apparatus.