A reliable synchronization apparatus and method within a multicast and broadcast service (MBS) zone having m sync executers (SEs) belonging to n retransmission zones is provided. Each retransmission zone has a selected leader SE and other member SEs. The apparatus comprises a sync controller and the n selected leader SEs. When the Sync controller multicasts a sync ruler to all SEs in a retransmission zone, the leader SE that loses the Sync ruler sends a NACK message to request the sync controller to retransmit the sync ruler. An election is triggered for updating leader information and announcing election to all SEs. The member SE that loses the sync ruler sends a NACK message to request its leader or the Sync controller to retransmit the sync ruler.