Codeigniter web.config index.php rewrite inside a subdirectory

Hi,

My website folder structure is like below.

Code:

Application Root
|- administration
|--|- application
|--|- system
|--|- index.php
|--|- web.config
|
|- application
|- system
|- index.php
|- web.config

Two CI framework installed here.

1. site root
   
2. administration subdirectory
   

Here I am talking about administration subdirectory.

My web.config URL Rewrite is like below.

Code:

<rewrite>
            <rules>
                <rule name="Index">
                    <match url="^(.*)$" />
                    <conditions>
                        <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
                        <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
                    </conditions>
                    <action type="Rewrite" url="index.php/{R:1}" />
                </rule>
            </rules>
        </rewrite>

problem is, if I remove index.php i.e.

PHP Code:

$config['index_page'] = ''; 


from site_root/administration/application/config/config.php, any controller function shows 404 page not found. But if I place it i.e.

PHP Code:

[b]$config['index_page']='index.php'; 


it works fine.

I want to remove index.php from URL.

My question is,

1. How could I perform it through web.config ?
   
2. How could I perform it through .htaccess ?
   

I have asked this same question here but got no answer. I google it and found similar question here without any answer.

For your information, I am using CI 2.2.6

Regards.