A control device (2, 3) constituting at least a part of a control system, the control device comprising a plurality of units (10, 20, 40) connected to each other through a communication line, wherein each of the plurality of units comprises: a controller (100, 150, 200); a receiver (112, 210a, 220a) configured to receive data from another unit through the communication line, the data being configured as a frame that is constructed by a plurality of blocks (304, 310) having a same data size, the first block of the frame being a header portion which comprises information indicating a priority of the data comprised in the frame and information on a length of the frame; a transmitter (114, 210b, 220b) configured to transmit the data as the frame to another unit through the communication line; a generator (200,240) configured to generate the data to be transmitted to another unit, wherein the transmitter (114, 210b, 220b) is configured to transmit a second frame comprising the higher-priority data to be transmitted after transmitting a first frame up to a predetermined block boundary between two consecutive blocks (304, 310), when the higher-priority data to be transmitted is generated in the receiver or the controller while the transmitter (114, 210b, 220b) transmits the first frame; wherein the transmitter (114, 210b, 220b) is configured to interrupt the transmission of an untransmitted portion of the first frame in units of blocks until the transmission of the second frame is completed; and wherein the transmitter (114, 210b, 220b) is configured to resume the transmission of the untransmitted portion of the first frame in units of blocks immediately after sending the second frame without adding a header to the untransmitted portion of the first frame. a controller (100, 150, 200); a receiver (112, 210a, 220a) configured to receive data from another unit through the communication line, the data being configured as a frame that is constructed by a plurality of blocks (304, 310) having a same data size, the first block of the frame being a header portion which comprises information indicating a priority of the data comprised in the frame and information on a length of the frame; a transmitter (114, 210b, 220b) configured to transmit the data as the frame to another unit through the communication line; a generator (200,240) configured to generate the data to be transmitted to another unit, wherein the transmitter (114, 210b, 220b) is configured to transmit a second frame comprising the higher-priority data to be transmitted after transmitting a first frame up to a predetermined block boundary between two consecutive blocks (304, 310), when the higher-priority data to be transmitted is generated in the receiver or the controller while the transmitter (114, 210b, 220b) transmits the first frame; wherein the transmitter (114, 210b, 220b) is configured to interrupt the transmission of an untransmitted portion of the first frame in units of blocks until the transmission of the second frame is completed; and wherein the transmitter (114, 210b, 220b) is configured to resume the transmission of the untransmitted portion of the first frame in units of blocks immediately after sending the second frame without adding a header to the untransmitted portion of the first frame.