Provided is a network system which improves a security and prevents illegal use when providing services such as Internet banking services. A random graphic table (RMT) is issued to a user, and having text characters which a user inputs and figures which corresponds to the text characters, respectively, and which is unrelated to the text characters such as a photograph. A banking organization server (30) manages random graphic table data corresponding to the random graphic table (RMT), distributes data for input including a portion of the random graphic table data to a communication terminal device (10) when information is inputted, and executing a specification of information to be specified while comparing the data for input with the random graphic table (RMT).