OK, I think I found a solution:

In Joomla root index.php you should insert this lines written with bold style:

$host=$_SERVER["REQUEST_URI"];
// check if $_SERVER["REQUEST_URI"] is "/", if yes we go to a special page
if ( $host == "/" ) {
$location="www.yoursite.com/homepage.html";
header( "Location: $location" );
exit();
}

Of course you should change www.yoursite.com and homepage.html as you want to use!