Apache服务器子目录页面未显示

我在ubuntu 12.04系统上使用LAMP。当我浏览位于我的/var/www/文件夹中的任何子目录或子页面时，它显示为

Not Found
The requested URL /directory/subdirectory/filename was not found on this server.
Apache/2.2.22 (Ubuntu) Server at localhost Port 80

但是该文件已经存在于该子目录中。那么，有人能告诉我为什么会出现这个错误吗？任何帮助和建议都是值得赞赏的。感谢

我的/etc/apache2/sites-available/default文件像这个

<VirtualHost *:80>
    ServerAdmin webmaster@localhost
    DocumentRoot /var/www
    <Directory />
        Options FollowSymLinks
        AllowOverride All
    </Directory>
    <Directory /var/www/>
        Options Indexes FollowSymLinks MultiViews
        AllowOverride All
        Order allow,deny
        allow from all
    </Directory>
    ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/
    <Directory "/usr/lib/cgi-bin">
        AllowOverride All
        Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
        Order allow,deny
        Allow from all
    </Directory>
    ErrorLog ${APACHE_LOG_DIR}/error.log
    # Possible values include: debug, info, notice, warn, error, crit,
    # alert, emerg.
    LogLevel warn
    CustomLog ${APACHE_LOG_DIR}/access.log combined
    Alias /doc/ "/usr/share/doc/"
    <Directory "/usr/share/doc/">
        Options Indexes MultiViews FollowSymLinks
        AllowOverride None
        Order deny,allow
        Deny from all
        Allow from 127.0.0.0/255.0.0.0 ::1/128
    </Directory>
</VirtualHost>