Website ping itself every one second

I have a website on the IIS 6.0 Windows Server 2003.

I have some strange thing happening in the server logs.  

There is a record of an IP which calls the default website every one second.

And that IP is the IP of our website. So, it seems that the website pings itself every one second. But I have no idea why it is happening.

Any idea appreciated.

Thanks.