After an even opening 30 minutes of the Group B match, the two-time champions opened the scoring through Tanahashi before goals from Takefusa Kubo and Keito Nakamura increased the half-time advantage to three.
The second half brought more joy for Japan's forwards in a five-minute spell on 52 minutes as first Nakamura, then Tanahashi again and Toichi Suzuki increased the lead to six.
There was still time for Tanahashi to net his third in the final 10 minutes before Kubo scored his fourth of the tournament in injury time as Japan bettered their 7-0 win over Vietnam in their Group B opener.
