I changed it.now It works fine
web.configfile:
<authentication mode="Forms">
<forms timeout="30" loginUrl="~/entrance_before_paying.aspx" defaultUrl="Admin/Default.aspx" name=".ASPXFORMSDEMO" cookieless="AutoDetect" protection="All" >
<credentials passwordFormat="Clear">
<user name="elmiragolshanff@yahoo.com" password="elmira" />
</credentials>
</forms>
</authentication>
my on button_click event in entrance_before_paying.aspx page :
if (FormsAuthentication.Authenticate(TextBox1.Text.Trim(), TextBox2.Text.Trim()))
{
Response.Redirect("Admin/Default.aspx");
Session["user"] = "admin";
}
else{