The short answer
Don't pick one: test Google Japan and Yahoo! JAPAN with the same conversion definition, then reallocate budget on evidence.
One of the most common questions from brands entering the Japanese market is whether to advertise on Yahoo! JAPAN, Google Japan, or both.
Yahoo! JAPAN provides additional search and display inventory in Japan and may reach audiences differently from Google Japan.
Cost-per-click differs by keyword, audience, season, and auction conditions, so comparable campaigns should be tested with the same conversion definition.
Yahoo! JAPAN offers unique ad formats including Yahoo! Display Ads with native placements across Yahoo! News and other high-traffic properties.
When both platforms are relevant, test them with bounded budgets and review search terms, conversion quality, and measurement gaps before reallocating spend.