An apparatus captures packets transmitted between first and second devices in accordance with first and second communication protocols, where the first and second protocols each assign a value for identifying a packet transmitted between first and second devices. In a state in which a first packet transmitted from the first device to the second device has not been captured yet at a time point of capturing an acknowledgment packet, responsive to the first packet, including a first value, upon capturing a second packet assigned a second value indicating order earlier than the first value, the apparatus determines whether the second packet is the first packet that is captured later than the acknowledgment packet, based on a difference between third and fourth values respectively assigned to the second packet and a previous packet that is captured latest among packets transmitted from the first device and captured earlier than the acknowledgment packet.