Right wing Fetu'u Vainikolo scored two tries to lead Tonga to a 27-17 victory in the opener of the Pacific Nations Cup tournament on Saturday.
Vainikolo touched down the first two tries in the fifth and 12th minutes to give Tonga a 10-0 lead and eventually his side never looked back.
After flyhalf Fangatapu Apikotoa converted his second try for 12-0, the Japanese pulled one back through hooker Takeshi Kizu's try in the 20th minute.
But Apikotoa added one penalty goal and a conversion with Will Helu's try in the 36th minute to end the first half 22-5.
Japan scored another try through right centre Male Sau in the beginning of the second half, but prop Eddie Aholelei cancelled it with a try in the 44th minute before left flanker Hendrik Tui scoring a converted try to complete the scoring in the 63rd minute.
Defending champions Samoa have opted out but the United States and Canada will debut later on Saturday in Edmonton in the expanded Pacific Nations Cup, which is expected to boost "second tier" nations.
Tries: Tui, Sau, Kizu
Tries: Helu, Vainikolo 2, Aholelei
Cons: Apikotoa 2
Japan squad: Kensuke Hatakeyama, Masataka Mikami, Yusuke Nagae, Hiroshi Yamashita, Yusuke Aoki, Shota Horie, Takeshi Kizu, Shoji Ito, Shinya Makabe, Hitoshi Ono, Michael Broadhurst, Justin Ives, Michael Leitch, Hendrik Tui, Takashi Kikutani, Atsushi Hiwasa, Fumiaki Tanaka, Keisuke Uchida, Kosei Ono, Yu Tamura, Male Sau, Harumichi Tatekawa, Craig Wing, Yoshikazu Fujita, Kenki Fukuoka, Yuta Imamura, Hirotoki Onozawa, Ayumu Goromaru, Takuma Asahara, Ryuta Yasui.
Tonga squad: Eddie Aholelei, Peni Fakalelu, Sila Puafisi, Kama Sakalia, Taione Vea, Ilaisia Ma'asi, Elvis Taione, Josh Afu, Daniel Faleafa, Emosi Kauhenga, Lua Lokotui, Soni Masima, Viliami Fihaki, Paula Kaho, Nili Latu, Hale T Pole, Taniela Moa, Thomas Palu, Fangatapu Apikotoa, Kurt Morath, Suka Hufanga, Sione Piukala, Siale Piutau, Will Helu, Viliame Iongi, Fetu'u Vainikolo, Viliami Hakalo, Tevita Halaifonua.