Multiplex transmission method and a synchronizing method in multiplex transmission

When a message is transmitted, frame by frame, from any one (10) of a plurality of multiplex nodes to a common multiplex bus (MB) to which the multiplex nodes are connected, each of the multiplex nodes (20 and 30) determines that transmission of a frame therefrom is allowed when it detects a transmission permission signal (b, b') added to the frame after detection of and idle state of the multiplex bus, and starts transmitting a message data frame. This makes it possible to properly implement priority-based control according to the priority levels of the data frames irrespectively of variations in the reference clocks of the multiplex nodes. Further, receiving multiplex nodes perform frame synchronization at the rise of a special code (a) of a start code (SOM) of a message data frame, and then perform re-synchronization at the rise of a special bit pattern (b'') which includes a passive bit and a dominant bit. This prevents synchronization from being deviated and bit errors resulting from deviated synchronization, thus leading to higher reliability of multiplex transmission.