In an electronic control system for a construction machine, a governor control unit, an excavator body control unit, and an electric lever control unit are interconnected for transmission and reception of data. A universal communication line is connected to the governor control unit and is adapted for an interface in conformity with universal communication standards A dedicated communication line is connected to the excavator body control unit and the electric lever control unit and is adapted for an interface in conformity with dedicated communication standards different from the univeral communication standards. A communication relay control unit is used for, converting and transmitting communication data received from one of two systems of the communication lines to be in conformity with the communication standards of the other communication line.