A control device efficiently performs priority-dependent data transmission through a communication line and a control method aimed at the priority-dependent data transmission. The control device includes a transmitter that transmits data as a frame to another unit through the communication line and a generator that generates the data to be transmitted to another unit. When the higher-priority data to be transmitted is generated in a receiver or a controller while the transmitter transmits a first frame, the transmitter transmits a second frame including the higher-priority data to be transmitted after transmitting a first frame up to a predetermined block.