In a priority determination apparatus (10), a dissimilarity index calculation unit (11) calculates a dissimilarity index between a transmission/reception performance record of a first traffic flow related to a first security alert notified from a network-type intrusion detection apparatus and a transmission/reception performance record of a second traffic flow related to a second security alert notified from the network-type intrusion detection apparatus in the past, the network-type intrusion detection apparatus being configured to detect an attack on an apparatus in a network. A priority determination unit (12) determines a priority of the first security alert based on the dissimilarity index calculated by the dissimilarity index calculation unit (11).