PURPOSE: To measure the size of a body located at the remote position, by measuring visual angles and the size of an image when the body at the remote position is observed through a lens system.

CONSTITUTION: When a body, whose size is expressed by l=L/2, is observed by a lens system, the size is given by l=n tan θ1 and l=m tan θ2 and a magnification α is given by tan θ2 /tan θ1, where θ1 is the visual angle corresponding to the length l when the magnification of the lens system is 1, θ2 is the visual angle when the magnification of the lens system is α, and (n) and (m) are the distance between the body and the lenses. At this time the size 2l of the body can be obtained by measuring the image sizes 2a and 2b on a gage surface separated by specified distances from the lens system and θ1 and θ2. When the magnification is 120, the size 2l of the body located at the remote position separated by about 100m to about 6.9km can be measured.