1. A method of providing a service among heterogeneous services, the method comprising: verifying that a second web application associated with an external web service is installed in a user agent, in response to data of the external web service being requested by a first web application executed in the user agent; requesting, from a first server providing a web service associated with the first web application, key information for accessing the requested data in response to the verifying that the second web application is installed in the user agent; receiving the key information from the first server; and accessing, in the first web application, the requested data of the second web application, using the received key information, wherein the key information is generated by the second web application, transferred to a second server providing the external web service, and delivered from the second server to the first server at a request of the first server.