To impose a restriction on actual use, such as playback, of content data and to reduce or prevent unauthorized use of content data by a unit that is not registered with an information provider, in response to an operation request to play content data recorded in a user unit, the user unit transmits current time information, the unit ID, content information, and operation information to a service center and, using these pieces of information, creates a one-time password that is valid only for a predetermined period of time. On the basis of the information transmitted from the user unit and current time information obtained from a time keeping unit of the service center, the service center similarly creates a one-time password and an operation permission command and transmits them to the user unit. The user unit compares the two one-time passwords. When the two one-time passwords match each other, the user-requested operation (playback) is executed.