PURPOSE: To secure the simultaneity and the relation among the data which are transmitted in a multiplex communication system for vehicles where the information are transferred among plural controllers mounted on the vehicles via a transmission line.

CONSTITUTION: A controller 3A stores the data which are sent to a data storage area and also stores the data requiring the simultaneity and the address value of the data consisting of plural bytes in a common data defining area independently of other data. Then the controller 3A transfers the data equivalent to a single frame and stored in the data storage area to a transmission controller 4 and then to a transmission line 2 to transfer the data equivalent to the next one frame to the transfer 4 based on the contents of the common data defining area. The data requiring the simultaneity and the data consisting of plural bytes are arranged in the same frame.