A plurality of electric control units are connected to perform data-communication through a communication line. Each electric control unit comprises a recording means for recording data and a communication means, to control an operation of an object to be controlled. One of the plurality of electric control units determines whether it is possible to record data in the recording means, and records the data in the recording means when it determines that it is possible to record the data in the recording means. The “one of the plurality of electric control units” transmits the data to other electric control unit when it determines that it is not possible to record the data in the recording means. The “other electric control unit” receives the transmitted data and records it in own recording means.