[Objective] To provide a technique for allowing, in a network communication system consisting of a first server for sending contents to a client and a second server for executing an application upon being called from the client, the first server to appropriately detect that the second server is called.[Solution Means] In a communication method in a network communication system composed of a first server, a second server, and a client, when predetermined contents are sent to the client from the first server, API information for executing an application program of the second server is included in the contents, and the application is executed on the second server by an API call from the client. During this, the second server generates execution certification information on the basis of dynamic common information that changes dynamically, and sends to the client the execution certification information along with the application execution result. The client sends the execution certification information to the first server, and the first server detects the execution of the application program on the second server.