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 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, the position of the center of the first area and the position of the center of the second area are both away from the 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, and three-dimensional position information about an observed object is acquired by the predetermined computation.