An operation unit detects that a received first ACK packet is an ACK packet transmitted in response to a single data packet. The operation unit determines whether to calculate a round trip time by using reception time of the first ACK packet on the basis of a window size included in the first ACK packet and a window size included in a second ACK packet acquired immediately before the first ACK packet. If the operation unit determines to calculate the round trip time, the operation unit outputs the round trip time calculated by using the reception time of the first ACK packet.