The attacks may have targeted the Japanese websites in protest against Tokyos resumption of whale hunting on December 1.
The trouble occurred at a time many people were using the site to prepare for upcoming annual tax declarations.
The unidentified person also threatened more such attacks and refused to reveal his or her identity.
Commercial whaling has been banned across the world for nearly three decades.
Despite international criticism, Japanese whale and dolphin hunting has continued.
source: www.techworm.net
