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; calculating, for each mesh in the adjacent mesh, a first value by multiplying the support and the confidence; calculating, for each mesh, in the adjacent mesh, a second value by dividing the first value by a minimum support among the supports of the meshes; excluding first meshes included in the adjacent mesh set from the adjacent mesh set, the first meshes having the second value which is smaller than the confidence threshold value; and repeatedly executing each process.