A plant data classification device according to one aspect of the invention includes: a data classification unit that classifies multidimensional operation data into categories; an evaluation index calculation unit that calculates an evaluation index of a category from a value of the operation data; a classification result evaluation unit that calculates a variation in the evaluation index for each category and determines whether the variation in the evaluation index is less than or equal to a reference value; and a parameter changing unit that changes, when it is determined that the variation in the evaluation index exceeds the reference value, a value of a parameter that defines a size of a category of the data classification unit in a direction of decreasing the size of the category.