A diagnosis apparatus 1 including: a target vibration information generation unit 2 that generates pieces of target vibration information; a comparative relationship information generation unit 3 that generates pieces of comparative relationship information indicating relationships among the pieces of target vibration information, or the pieces of comparative relationship information indicating relationships among pieces of reference vibration information and the pieces of target vibration information, the pieces of reference vibration information indicating vibrations in the target mode among vibrations that are measured in a reference period; and a diagnosis unit 5 that diagnoses that a change has occurred in the structure 20 between the diagnosis period and the reference period based on a result of comparison between a distribution of pieces of reference relationship information and the distribution that has been calculated in relation to the pieces of comparative relationship information.