Although Yahoo Japan! is still the typical search engine of choice, Google's search volumes and market share in the Japanese market have been improving markedly.
As a result of this, Yahoo's control over the Japanese search engine market domain has been in decline. Usage statistics below display this information:

2007 May
Yahoo! Japan 63.3%
Google Japan 22.4%
Google com (USA) 5.9%
MSN 4.0%
Other 4.4%

2008 Jan
Yahoo! Japan 60.4%
Google Japan 24.7%
Google com (USA) 7.1%
MSN 3.6%
Other 4.2%

Reference:
http://www.webcreate.ga-pro.com/search.html