Subdomain redirecting to folder

I installed WordPress in a subdomain http://blog.edirpedro.com.br/

This is how .htaccess on root domain point this subdomain to the folder:

Options +FollowSymLinks -MultiViews

RewriteEngine on
RewriteBase /
RewriteCond %{HTTP_HOST} ^(www\.)?blog\.edirpedro\.com\.br$ [NC]
RewriteCond %{REQUEST_URI} !^/blog/
RewriteRule ^(.*)$ /blog/$1 [L]

But WordPress is getting the foldername and use it. When I try to access / wp-admin/ it redirects to http://blog.edirpedro.com.br/wp-login.php?redirect_to=http://blog.edirpedro.com.br/blog/wp-admin/, notice the /blog/ on the redirect url. This is blocking login page and is causing other problems in WordPress, for many times I got a 500 server error page.

Could someone help me here?
I’m getting crazy because I’ve already read the Mode Rewrite manual, tried to find some solution on web and with my host, but still nothing work.