She said late Thursday that Japan hoped Iran would continue to cooperate with the International Atomic Energy Agency (IAEA).
Iran signed the additional protocol to the nuclear Non-Proliferation Treaty at the Vienna headquarters of the IAEA.
"We hope (Iran) would ratify and fully implement" its commitments under an additional nuclear protocol, Kawaguchi said in a statement.
"We also hope it will continue to cooperate with IAEA during the intervening period leading to the ratification under the additional protocol," she said.
Iran's move capped months of intense diplomacy to pressure Tehran into placing its nuclear facilities under greater international supervision and prove that it is not seeking to develop atomic weapons.