A data transmission device including a network interface unit (1, 5) having means for carrying out a process of transmitting and receiving data to and from a network, and means for detecting data which are being transmitted in said network and notifying to a monitoring unit an identifier assigned to each said data being transmitted and a transmission timing of each said data being transmitted; said transmission device comprising: a monitoring unit having means for recording into a table of a recording unit the transmission timing of the data notified from said network interface unit according to the identifier, for monitoring the transmission time period of said data having the same identifier, for calculating a transmission time period of said data having the said identifier, and for predicting a next transmission timing of said data having the same identifier by using said transmission time period; a recording unit being a memory in which a table showing the transmission time period, the newest transmission timing, and a predicted next transmission timing at which data will be transmitted next time is recorded according to the identifier of the data; and a transmission timing control unit (4, 6) for delaying a transmission timing of data in the data transmission device when there is a particular predicted next transmission timing among the predicted next transmission timing recorded in said table in said recording unit, the particular predicted next transmission timing being the one whose time difference from the transmission timing of data in the data transmission device is within a threshold, and for commanding said network interface unit (1, 5) to transmit the data in the data transmission device when there is not said particular predicted next transmission timing among the predicted next transmission timing recorded in said table in said recording unit. a monitoring unit having means for recording into a table of a recording unit the transmission timing of the data notified from said network interface unit according to the identifier, for monitoring the transmission time period of said data having the same identifier, for calculating a transmission time period of said data having the said identifier, and for predicting a next transmission timing of said data having the same identifier by using said transmission time period; a recording unit being a memory in which a table showing the transmission time period, the newest transmission timing, and a predicted next transmission timing at which data will be transmitted next time is recorded according to the identifier of the data; and a transmission timing control unit (4, 6) for delaying a transmission timing of data in the data transmission device when there is a particular predicted next transmission timing among the predicted next transmission timing recorded in said table in said recording unit, the particular predicted next transmission timing being the one whose time difference from the transmission timing of data in the data transmission device is within a threshold, and for commanding said network interface unit (1, 5) to transmit the data in the data transmission device when there is not said particular predicted next transmission timing among the predicted next transmission timing recorded in said table in said recording unit.