The farm ministry said Friday that ducks at the Imperial Household Agency's duck farm in Koshigaya, Saitama Prefecture, have become infected with an attenuated avian influenza virus.
The Agriculture, Forestry and Fisheries Ministry said the birds do not have to be culled and asked the agency to monitor them. All birds on the farm would have been culled if they had been infected with a highly pathogenic version of bird flu.
The farm is used by the Emperor to entertain guests from Japan and abroad.