A position detector displays a target on a given plane and adds a standard on the given plane in the vicinity of the target with the location of the standard known. An image of the given plane is formed on an image plane of an image sensor with an image of the standard included, a point in the image of the given plane which is formed at a predetermined position of the image plane corresponding to the point to be detected. An image processor identifies the image of the standard on the image plane to calculate the position of the point to be detected. The standard includes asymmetric pattern. The standard includes a first standard and a second standard sequentially added on the given plane, the difference being calculated accompanied with the plus sign or the minus sign. The image on the given plane is formed by means of a scanning, the image sensor reads out the sensed image upon the termination of at least one period of the scanning. The second standard is added upon the initiation of the scanning after the completion of reading out of the image of the first standard.