Call Web Service through SSL

I have an asp.net web site which need to call a secured web service.
In the web.config of the web site, the web service URL is configured as
<appSettings>
  <add key="DataService.DataService" value="https://MyServer/MyService/DataService.asmx"/>
 </appSettings>
But I got the following excepton:

Exception: System.Net.WebException: The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.
If the web service does not use SSL, everything is OK with configuration http://MyServer/MyService/DataService.asmx.
Can any one tell me what the problem is? Thanks.
 
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