Give yahoo.co.jp an IPv6 address Ask your hosting provider for the site's IPv6 address (or the hostname to point at), then publish it as an AAAA record. If your host doesn't offer IPv6, a proxy that does can sit in front. How to add an AAAA record
www.yahoo.co.jp is an alias of edge12.g.yimg.jp, which has no IPv6 www.yahoo.co.jp is a CNAME, so it can't also have its own AAAA record. Ask whoever runs edge12.g.yimg.jp to enable IPv6 there — or replace the alias with the A and AAAA records they give you. Why some hosts don't offer IPv6
Mail servers without IPv6 mx5.mail.yahoo.co.jp, mx1.mail.yahoo.co.jp, mx2.mail.yahoo.co.jp, mx3.mail.yahoo.co.jp have no AAAA record, so mail from IPv6-only servers can't reach them directly. If a provider runs your mail, that's their setting; if you run it, add AAAA records for these hosts.
Nameservers without IPv6 ns02.yahoo.co.jp, ns01.yahoo.co.jp, ns12.yahoo.co.jp, ns11.yahoo.co.jp all have no AAAA record, so a resolver that only speaks IPv6 can't look your domain up. Most DNS providers offer IPv6 nameservers; ask yours, or move DNS to one that does.
Visitors on IPv6-only networks (many mobile carriers) can still reach it, but only through a translator their carrier runs (NAT64). The site itself has no IPv6 address — and you can't make one up from the IPv4. Why some hosts don't offer it
Copy the evidence— a citation-ready record of this measurement