PURPOSE: To detect the position of a coil in an objective accuracy scope by specifying the area the tail end of the coil is leaped up, and providing a tail end removing device to recognize the result of calculation of a parameter of the coil from the height distribution data in the diameter direction on the coil including no heaping-up area.

CONSTITUTION: A coil is photographed by TV cameras 1 and 2 in the condition laser beams are not radiated, and after A-D converted by an A/D converter 7, it is stored in a standard image memory 8. Then, a scan mirror 6 is controlled and a slit light parallel to the axis is radiated on the coil. The coil is photographed by the TV camera 1, and after A-D converting by the A/D converter 7, the difference from the standard image memory 8 is calculated by a binary process device 9, and after that, a binary process is applied by a fixed threshold value to obtain an image data. After converting it in the X, Y, Z coordinates system by a coordinates converting device 10, the data from which the leaping-up component at the tail end is removed is substituted in an equation of a circle by a tail end removing device 16 and an operation device 11. By solving using the Newton-Raphson method, the center and the rudius of the coil is found, and the data is delivered to a controller 15.