A communication method in a network communication system composed of a first server, a second server, and a client connected via a network, comprising: a contents transmission step of allowing a contents transmission means of the first server to send to the client predetermined contents including at least application programming interface, API, information for executing a predetermined application program stored in the second server; a contents reception step of allowing a contents reception means of the client to receive the contents; an application programming interface, API, call step of allowing an application programming interface, API, call means of the client to call the second server based on the application programming interface, API, information; an application execution step of allowing an application execution means of the second server to execute, depending on the call, the predetermined application program stored in a second storage means; an execution certification information generation step of allowing an execution certification information generation means of the second server to generate execution certification information based on dynamically-changing dynamic common information, wherein the dynamically-changing dynamic common information is information shared among the first server and second server which information is different at least at a different time and changes by an arbitrary trigger or interval; an application programming interface, API, response step of allowing an application programming interface, API, response means of the second server to send the application programming interface, API, execution result of the application execution means and the execution certification information to the client; an application programming interface, API, execution result reception step for allowing the contents reception means of the client to receive the application programming interface, API, execution result and the execution certification information; an execution certification information transmission step of allowing the execution certification information transmission means of the client to send at least the execution certification information to the first server; an execution certification information reception step of allowing the execution certification information reception means of the first server to receive at least the execution certification information from the client; and an application programming interface, API, execution detection step of allowing the application programming interface, API, execution detection means of the first server to detect, based on the execution certification information, the execution of the predetermined application program by the application execution means of the second server in association with the contents transmission by the contents transmission means. a contents transmission step of allowing a contents transmission means of the first server to send to the client predetermined contents including at least application programming interface, API, information for executing a predetermined application program stored in the second server; a contents reception step of allowing a contents reception means of the client to receive the contents; an application programming interface, API, call step of allowing an application programming interface, API, call means of the client to call the second server based on the application programming interface, API, information; an application execution step of allowing an application execution means of the second server to execute, depending on the call, the predetermined application program stored in a second storage means; an execution certification information generation step of allowing an execution certification information generation means of the second server to generate execution certification information based on dynamically-changing dynamic common information, wherein the dynamically-changing dynamic common information is information shared among the first server and second server which information is different at least at a different time and changes by an arbitrary trigger or interval; an application programming interface, API, response step of allowing an application programming interface, API, response means of the second server to send the application programming interface, API, execution result of the application execution means and the execution certification information to the client; an application programming interface, API, execution result reception step for allowing the contents reception means of the client to receive the application programming interface, API, execution result and the execution certification information; an execution certification information transmission step of allowing the execution certification information transmission means of the client to send at least the execution certification information to the first server; an execution certification information reception step of allowing the execution certification information reception means of the first server to receive at least the execution certification information from the client; and an application programming interface, API, execution detection step of allowing the application programming interface, API, execution detection means of the first server to detect, based on the execution certification information, the execution of the predetermined application program by the application execution means of the second server in association with the contents transmission by the contents transmission means.