○○.com
　┣index.html
　┣mobile
　┣　┣index.html
　┣　┗.htaccess
　┗.htaccess

という形で、○○.comの下の.htaccessに

RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} .*docomo.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*j\-phone.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*vodafone.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*kddi.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*pdxgw.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*opmv.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*mmp.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*ginga.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*ddipocket.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*astel.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*l\-mode.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*up\.browser.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*lynx.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*Hidemarnet.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*w3m.* [NC]
RewriteRule ^index\.html$ mobile/index.html [L]

と記述し、mobileの下の.htaccessに
RewriteEngine off
と記述しました。