The　masses　of　the　three　generations　of　charged　leptons　are　known　to　completely　satisfy　Koide＇s　mass　relation,　but　the　question　remains　of　whether　such　a　relation　exists　for　neutrinos.　In　this　paper,　by　considering　the　seesaw　mechanism　as　the　mechanism　generating　tiny　neutrino　masses,　we　show　how　neutrinos　satisfy　Koide＇s　mass　relation,　on　the　basis　of　which　we　systematically　give　exact　values　of　both　left-　and　right-handed　neutrino　masses.