A communications apparatus, enabling an efficient data transfer without an environmental restriction, notifies a transmission-side apparatus (200) connected via a network (300) 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 (200). The communications apparatus includes: a receiving buffer memory (120) having a region for storing a data packet transmitted from the transmission-side apparatus (200); a communications protocol processing unit (130) notifying the transmission-side apparatus (200) of the window size larger than a capacity of the receiving buffer memory (120), receiving the data packet transmitted from the transmission-side apparatus (200) according to the notified window size, and storing the received data packet in the receiving buffer memory (120); and a central controlling unit (140) retrieving the data packet stored in the receiving buffer memory (120) and processing the retrieved data packet.