A packet analysis apparatus analyzes content of communication obtained as a result of monitoring or capturing a packet passing through a network. The apparatus has a unit of acquiring source or destination address information from a network layer packet header. The apparatus has a unit of acquiring from the network layer packet header an identifier for which a value that increases monotonously with each sending for each source or destination address information is set. The apparatus has a unit of searching and acquiring an identifier corresponding to address information in a current packet from a storage part holding an identifier in a previous packet corresponding to source or destination address information. The apparatus has a unit of comparing the identifier in the previous packet acquired and the identifier in the current packet and determining that reordering occurs when the identifier in the current packet is smaller.