how to pass data with query string by using window.locatin

Hi Everybody,
firstly I wanna show alert message and then i wanna go to another page by passing data with query string. My code show me alert message and go to another page but it doesn't carry value with query string.
Here is my code....
 string message = "Room Check Out Successful";  ScriptManager.RegisterStartupScript(this.Page, this.GetType(), "checkoutalart", "alert('" + message + "'); window.location='Billing.aspx?billingNo=+billingNo';", true);
please give me some advice...
Thanks....