A priority verification unit (110) obtains from, among a plurality of communication apparatuses, other communication apparatus other than a current grandmaster, a grandmaster mediation message that is a grandmaster mediation message used for selecting a grandmaster and that includes a priority of the other communication apparatus. The priority verification unit (110) verifies whether or not the priority of the other communication apparatus is higher than a grandmaster priority (22). A mediation unit (120), when the priority of the other communication apparatus is verified as being higher than the grandmaster priority (22), to obtain a synchronization message including time of the other communication apparatus from the other communication apparatus before switching the other communication apparatus to the grandmaster. The mediation unit (120) calculates difference between the time of the other communication apparatus and time of the communication apparatus, and when the difference is less than a threshold (23), switches the other communication apparatus to the grandmaster.