1. A method of providing a service among heterogeneous services, the method comprising: verifying whether a second web application associated with an external web service is installed in a user agent, when data of the external web service is requested from a first web application executed in the user agent; requesting key information to be used for accessing the requested data from a first server providing a web service associated with the first web application, when the second web application is installed in the user agent; receiving the key information from the first server; and accessing the requested data existing on the second web application, using the received key information, in the first web application.