Plural communication areas are provided, and plural electronic control units for mutually making communication in each of the communication areas are provided. A vehicular power line communication system includes a power line coupling the respective communication areas to one another, branch power lines coupling the respective electronic control units in each of the communication areas to one another, and a battery for supplying electric power to the electronic control units through the power line and the branch power lines. The power line is formed of a +B line turning to a positive polarity and a ground line turning to a negative polarity, which are cabled to be spaced from each other in a part of the power line. The branch power lines superpose communication data thereon, thereby making data communication.