A communications apparatus, enabling an efficient data transfer without an environmental restriction, notifies a transmission-side apparatus connected via a network of data amount in a window size, and receives data as large as the window size, the data which is transmitted from the transmission-side apparatus. The communications apparatus includes: a receiving buffer memory having a region for storing a data packet transmitted from the transmission-side apparatus; a communications protocol processing unit notifying the transmission-side apparatus of the window size larger than a capacity of the receiving buffer memory, receiving the data packet transmitted from the transmission-side apparatus according to the notified window size, and storing the received data packet in the receiving buffer memory; and a central controlling unit retrieving the data packet stored in the receiving buffer memory and processing the retrieved data packet.