A round-trip delay time measurement system comprising a first communication device and a second communication device, wherein the first communication device comprises: a measurement packet transmission unit that transmits a train of measurement packets, starting at a first transmission time point; a return packet reception unit that receives a return packet at a second reception time point, the return packet including information relating to a processing time indicating a duration from a first reception time point of a first measurement packet in the train of measurement packets to a second transmission time point, the return packet having a same packet size as the first measurement packet in the train of measurement packets; and a delay time specification unit that specifies, as a round-trip delay time, a duration obtained by subtracting the processing time from a duration from the first transmission time point to the second reception time point, and the second communication device comprises: a measurement packet reception unit that receives the train of measurement packets, starting at the first reception time point; a return time point determination unit that determines the second transmission time point at which the return packet is transmitted after the complete reception of the train of measurement packets; a processing time specification unit that specifies the information relating to the processing time; a return packet generation unit that generates the return packet; and a return packet transmission unit that transmits, at the second transmission time point determined by the return time point determination unit, the return packet as a reply to the first measurement packet in the train of measurement packets. a measurement packet transmission unit that transmits a train of measurement packets, starting at a first transmission time point; a return packet reception unit that receives a return packet at a second reception time point, the return packet including information relating to a processing time indicating a duration from a first reception time point of a first measurement packet in the train of measurement packets to a second transmission time point, the return packet having a same packet size as the first measurement packet in the train of measurement packets; and a delay time specification unit that specifies, as a round-trip delay time, a duration obtained by subtracting the processing time from a duration from the first transmission time point to the second reception time point, and the second communication device comprises: a measurement packet reception unit that receives the train of measurement packets, starting at the first reception time point; a return time point determination unit that determines the second transmission time point at which the return packet is transmitted after the complete reception of the train of measurement packets; a processing time specification unit that specifies the information relating to the processing time; a return packet generation unit that generates the return packet; and a return packet transmission unit that transmits, at the second transmission time point determined by the return time point determination unit, the return packet as a reply to the first measurement packet in the train of measurement packets. a measurement packet reception unit that receives the train of measurement packets, starting at the first reception time point; a return time point determination unit that determines the second transmission time point at which the return packet is transmitted after the complete reception of the train of measurement packets; a processing time specification unit that specifies the information relating to the processing time; a return packet generation unit that generates the return packet; and a return packet transmission unit that transmits, at the second transmission time point determined by the return time point determination unit, the return packet as a reply to the first measurement packet in the train of measurement packets.