Vehicle-mounted device and congestion control method

A congestion control method of a vehicle-mounted device that performs communication by radio signals with predetermined vehicle-mounted devices respectively mounted in a plurality of vehicles, comprising:

• detecting said radio signals to measure congestion level;
• receiving, from each of said predetermined vehicle-mounted devices, a value of, among parameters relating to communication of the predetermined vehicle-mounted devices, a dominant parameter for which degree of contribution to congestion increases with increase or decrease in the value of the dominant parameter;
• and
• when said congestion level measured exceeds a predetermined threshold value, changing the value of said dominant parameter of its own device in a direction that prevents occurrence of congestion when the value of said dominant parameter of its own device deviates in a direction that causes the congestion from an average of values of said dominant parameter that are received,
• wherein the dominant parameter is a predetermined parameter for which degree of contribution to congestion increases with increase in the value of the predetermined parameter, and
• when said congestion level measured exceeds a predetermined threshold value, decreasing the value of said predetermined parameter of its own device when the value of said predetermined parameter of its own device greater than an average of values of said predetermined parameter that are received.