The present invention provides a communication master station startup period control method of being able to securely prevent malfunction of a PLC system, even if at least two communication master stations are connected to the PLC system which permits only one communication master station on the network. After the startup, when it is determined that the communication master stations exist redundantly by a master plurality determination process performed in advance of a link establishment process, the transition to a predetermined master plurality avoidance process is made without making the transition to the link establishment process. Further, when it is determined that the communication master stations exist redundantly by the master plurality determination process performed in parallel with the link establishment process, the transition to the predetermined master plurality avoidance process is made without making the transition to an operation state.