Doubt on authentication

[smitha nair]

Hi
In my tomcat userrs file:-
<tomcat-users>
<role name="Guest"/>
<role name="Member"/>
<role name="Admin"/>
<user name="Bill" password="member" roles="Member"/>
<user name="sham" password="admin" roles="Admin,Guest"/>
<user name="mini" password="guest" roles="Guest"/>
</tomcat-users>
In web.xml:-
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd" version="2.4">
<servlet>
<servlet-name>Check_Authentication</servlet-name>
<servlet-class>com.example.Check_Authentication</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>Check_Authentication</servlet-name>
<url-pattern>/Check_Authentication</url-pattern>
</servlet-mapping>
<security-role>
<role-name>Admin</role-name>
</security-role>
<security-constraint>
<web-resource-collection>
<web-resource-name>check</web-resource-name>
<url-pattern>/Check_Authentication</url-pattern>
<http-method>POST</http-method>
</web-resource-collection>
<auth-constraint>
<role-name>Admin</role-name>
</auth-constraint>
</security-constraint>
<login-config>
<auth-method>FORM</auth-method>
<form-login-config>
<form-login-page>/loginPage.html</form-login-page>
<form-error-page>/loginError.html</form-error-page>
</form-login-config>
</login-config>
</web-app>
when i request this source which used post method.i get the custom form atuhentication page ,but when i give username and password it is not recognizing and shows error page instead of going to the resource(servlet).
please someone tell what is the problem..