Werder Bremen striker Yuya Osako scored a hat trick, while Liverpool midfielder Takumi Minamino – who is currently on loan at Southampton – was also on the scoresheet.
Mongolia, ranked 190th in the world, recorded just two shots against 34 in Japan.
It was Japan’s second-biggest win, only behind a 15-0 victory over the Philippines in 1967.
The victory means Japan will advance to the next phase of the Asian World Cup qualifying tournament if they beat Myanmar in the teams meeting on June 3.
After starting the five-goal break at half-time, the second period saw nine more goals for the Samurai Blue – four of them in just 11 minutes.
Japan’s other scorers were Daichi Kamada, Hidemasa Morita, Takuma Asano, an own goal from Khash Erdene Tuyaa and accolades from Sho Inagaki, Junya Ito and Kyogo Furuhashi.
Despite being a home game for Mongolia, the game took place at an empty Fukuda Denshi Arena in Chiba, Japan, after Mongolia had to relinquish their hosting rights due to quarantine regulations.
It wasn’t the first one-sided result of the week as Canada beat the Cayman Islands 11-0 on Monday in yet another World Cup qualifier.
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