Two wells are selected and upper, lower, left, and right logical edge coordinates of the two wells are calculated. An image of a portion near each set of logical edge coordinates is captured, and a rectangular separability filter is applied to each captured image and for each captured image, coordinates of a center position of the rectangular separability filter obtained when a peak value of a separability is obtained are detected as temporary edge coordinates. Thereafter, actual center coordinates are calculated for each well selected, based on upper, lower, left, and right temporary edge coordinates. Finally, the amount of positional displacement of a well plate from an ideal placement state is calculated based on the center coordinates of the two wells selected.