In a retransmission-request transmitting method, the receiving side apparatus activates a reordering timer, when receiving a first packet before receiving an unreceived packet with a sequence number smaller than a sequence number of the first packet; triggers transmission of a retransmission request for the unreceived packet, when having not received the unreceived packet by the time of expiration of the reordering timer activated in response to the receipt of the first packet; and stops and reactivates the reordering timer activated in response to the receipt of the first packet, when a value of the sequence number of the first packet falls out of a range of the receiving side window as a result of changing the upper limit value and the lower limit value in accordance with a sequence number of a second packet received from the transmitting side apparatus.