According to Hawai‘i Tourism Japan, as long as the visitor isolated for 5 days AND received a negative test AND acquired the Japan government standardized PCR negative proof form, they can travel back to Japan.
If the test result is still positive, another PCR test must be taken in the following days. This will continue until a negative result is obtained.
We suggest contacting the Japanese Consulate  and your airline to get up-to-date information.