1. An optical position detector for detecting the position of at least one object point, comprising receiving means for receiving a light emitted from at least one object point, comprising image forming means, a scale plate, and image pickup means, each being disposed along the same optical axis; and arithmetic operation means connected to the image pickup mean; wherein the scale plate has a plurality of pattern elements arranged periodically along a front coordinate plane perpendicularly intersecting the optical axis, the image forming means includes means for condensing a light beam emitted from an object point, converting the beam into a corresponding reference light converging into a corresponding image forming point which is displaced from the front coordinate plane of the scale plate, spot-illuminating a part of the scale plate with the and projecting at least one illuminated pattern element in a magnified scale, the image pickup means has a light receiving surface which is disposed along a rear coordinate plane perpendicularly intersecting the optical axis and which picks up a shadow of the at least one illuminated pattern element projected in the magnified scale and includes means for outputting corresponding image data, and the arithmetic operation means includes means for processing the image data, deriving a rear coordinate plane value of the shadow of the at least one illuminated pattern element, determining a position of the image forming point for the reference light on the basis of the respective rear coordinate plane value and a front coordinate plane value of the at least one illuminated pattern element, and determining a position of the object point on the basis of the position of the image forming point.