A position measurement system includes a photographing unit and an
arithmetic processing unit. The photographing unit has a lens and a
light-receiving element. The lens forms an optical ring image from light
from a light source through spherical aberration. The light-receiving
element detects the optical ring image formed by the lens. The arithmetic
processing unit measures a distance to the light source on the basis of
the quantity of light of the optical ring image detected by the
light-receiving element.