To calculate roundness easily without being influenced by dust adhesion of the like.

A computer 5 takes an image of an inspection object W imaged by an imaging camera 4, specifies the center point of a regression circle of a circular circumference of the inspection object W, calculates each distance from the center point to each point on the circular circumference, acquires the minimum outer radius from the minimum value among them, and acquires an average outer radius by calculating a mean value of the distances from the center point to each point on the circular circumference, to thereby calculate the roundness of the circumference of the inspection object based on a difference between the average outer radius and the minimum outer radius.