A data processing device connected to another data processing device through an asynchronous network to receive/transmit data from/to the other data processing device. The data processing device monitors an accumulation amount of a reception buffer for receiving data transmitted from the other data processing device, controls a self reception data processing clock based upon the monitored accumulation amount, transmits to the other data processing device transmission clock control information controlling a transmission clock in the other data processing device based upon the monitored accumulation amount, and controlling a self transmission clock based upon the clock control information transmitted from the other data processing device.