1. A method of calculating a surface shape of a target surface with a calculation apparatus, the method comprising: a measurement data obtaining step of obtaining measurement data of the surface shape of the target surface within each of a plurality of measurement target regions, each of the plurality of measurement target regions being set to partially overlap another measurement target region on the target surface, central positions of the measurement target regions being different from each other; an average data generating step of generating average data by dividing a result, obtained by adding the measurement data of the surface shape of the target surface within each of the plurality of measurement target regions obtained in the measurement data obtaining step while matching the central positions of the measurement target regions, by the number of the measurement target regions to be added; a subtracting step of subtracting the average data generated in the generating step from the measurement data of the surface shape of the target surface within each of the plurality of measurement target regions obtained in the measurement data obtaining step; and a calculating step of calculating data of the surface shape of the target surface in each measurement target region using data obtained in the subtracting step, and calculating the surface shape of the target surface by combining the data of the surface shape of the target surface in each measurement target region, wherein the calculation apparatus has a processing unit that executes the measurement data obtaining step, the average data generating step, the subtracting step, and the calculating step.