cvc-elt.1: Cannot find the declaration of element 'web-app'

When I created new dynamic web project in Eclipse, automatically created web.xml showed the following error:

cvc-elt.1: Cannot find the declaration of element 'web-app'

and a red background on this line:

<web-app id="WebApp_ID" version="2.4" 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">

I want to know why this error occur and how to get rid of this error.I used Eclipse version 3.1, Apache Tomcat 5.0 and JDK 1.4 in my project. I'm pretty new to Eclipse.