When long polling is employed under HTTP, a content-synchronization control device can control each content playback device so that content control is synchronized in multiple content playback devices. When receiving an inquiry from a content playback device 90, the response control means 84 waits transmission of an inquiry response in reply to the inquiry for a certain period of time since reception of the inquiry, and when the waiting state of the transmission of an inquiry response is cancelled, the waiting state of transmission of all the inquiry responses are cancelled. After the reception of the request information, the response control means 84 cancels the waiting state of the transmission of each inquiry response when a condition that the transmission of each inquiry response to each content playback device is kept waiting is satisfied.