In the prior art, in a case of measuring a speed of a narrow-bandwidth path of which the state fluctuates dynamically, a serious influence is placed upon a speed calculation with a sufficient precision, and a performance.Embedding probe information into a data packet, being actual data, to transfer two at a time allows the data packet having the probe information embedded to act as a probe packet of the packet pair technique as well. Further, in a system including a plurality of the paths, the data packets are distributed to the path two packets by two packets.