1. A three-dimensional position information acquiring method comprising: acquiring an image of a first optical image; acquiring an image of a second optical image after acquiring the image of the first optical image; and performing a predetermined computation using image data of the first optical image and image data of the second optical image, wherein: acquisition of the image of the first optical image is performed based on light beams having passed through a first area, acquisition of the image of the second optical image is performed based on light beams having passed through a second area, a position of a center of the first area and a position of a center of the second area are both away from an optical axis of an optical system in a plane perpendicular to the optical axis of the optical system, the first area and the second area respectively include at least portions that do not overlap with each other, three-dimensional position information about an observed object is acquired by the predetermined computation, the first area and the second area are formed at rotationally symmetric positions, the predetermined computation performs subtraction processing using data of the first optical image and data of the second optical image, a subtracted image is generated by the subtraction processing, the method further comprises: dividing the subtracted image into a plurality of measurement areas, and performing Fourier transform in each of the measurement areas, and Fourier coefficients are calculated in each of the measurement areas by the Fourier transform.