A method is disclosed that works effectively even in the case where a tampering detection method has become known to the public or in the case where program code for executing detection itself has been tampered with. A vibration-information obtaining unit obtains, from a player terminal, vibration history information representing a time series of the results of detection by a vibration detecting unit of the player terminal. An operation-information obtaining unit obtains, from the player terminal, operation history information representing a time series of the results of detection by a touch-operation input unit of the player terminal. An operation-information validity determining unit determines the validity of the operation history information obtained from the operation-information obtaining unit by comparing the operation history information with the vibration history information for which the validity has been determined by a vibration-information validity determining unit.