The system for reprogramming a plurality of control units includes a first control unit and a second control unit connected by one CAN bus, and a diagnosing unit. The diagnosing unit includes a first controller configured to reprogram the first control unit and a second controller configured to reprogram the second control unit. The diagnosing unit is configured to perform bidirectional reprogramming for the first control unit and the second control unit while sharing information between the first controller and the second controller.