1. An observation information processing apparatus, comprising: a memory; and a processor coupled to the memory, wherein the processor executes a process comprising: calculating, for each mesh, a support that is a number of records for which a condition part of an association rule becomes true in records included in a mesh, based on meshes each associated with records each including observation information having information identifying a geographical position and an event that occurs at the geographical position, the information being associated with the event; calculating, for each mesh, a confidence that is a ratio of records for which a conclusion part of the association rule also becomes true, to the records for which the condition part becomes true; generating an adjacent mesh set by merging adjacent ones of the meshes; calculating a confidence for each adjacent mesh based on the support and confidence of each mesh included in the adjacent mesh set; setting the smallest one of the confidences calculated as a new confidence threshold value; excluding meshes to be excluded from meshes included in the adjacent mesh set, based on the confidences and supports of the meshes included in the adjacent mesh set and the confidence threshold value; and repeatedly executing each process.