A management center registers user information which includes user identification information for identifying a user terminal and improvement-information identification information for identifying improvement information recorded in an optical disk and processed by the user terminal. When a user terminal requests improvement information through a network, the management center searches for another user terminal which can provide necessary information, according to user information, and obtains the improvement information from the another user terminal searched for, through the network. The management center then provides the obtained improvement information for the user terminal which requested it, through the network.