PURPOSE: To decide fully automatically the position of a diagram and to perform measurement simply and quickly by scanning the surface of a drawing of a diagram, etc. on both X and Y axes by a photodetector and detecting the position of the diagram from the difference between the radiated light quantity and the reflected light quantity.

CONSTITUTION: A photodetector 2 of a coordinate reader 1 is supported by a support part 5 with a prescribed distance set from a drawing A so as to attain a scan. Then the detector 2 detects the difference between the radiated light quantity and the reflected light quantity for a diagram, etc. drawn on the drawing A. The detector 2 has reciprocating motions in the prescribed direction on the part 5 through the 1st shift part 11 and is shifted orthogonally to said direction by an amount equivalent to the detection width of the detector 2 through the 2nd shift part 15. The distances from the reference positions of the parts 11 and 15 are detected by the 1st and 2nd shift amount detecting parts 21 and 22. A coordinate deciding arithmetic part is provided to an electric circuit of the reader 1 to compare and operate the signals sent from the parts 21 and 22 with the signal given from the detector 2. Thus the position of a diagram is decided fully automatically, and the measurement is simplified.