PURPOSE: To detects marks on an object positively by scanning the surface of the object by means of an optical sensor and reading predetermined number of existing marks in the order of contrast as marks.

CONSTITUTION: An object to be inspected, i.e., a circuit board 1, secured onto an XY table 7 is moved back and forth and to the right and left by means of a table 7 so that the main plane thereof is scanned by means of an optical sensor 3. A computor 10 previously fed with the number (n) (e.g. 5) of marks 2 receives an output from the sensor 3 subjected to A/D conversion 12 and an output from the table 7 representative of the coordinate of detecting point and delivers operation results to an output unit 11. The sensor 3 reads the quantity of reflected light while scanning the main plane of the substrate 1 and produces the results in the form of detection level of the quantity of reflected light. Low peak detection level represents a high contrast part and five existing marks 2 are detected in the order of contrast as marks 2 but sixth and subsequent marks are not detected. This constitution allows positive detection of predetermined number of marks.