An inspection system which includes a memory for storing image data provided by picking up an image of a workpiece, a monitor for displaying the image data stored in the memory on a display screen with pixels arranged in an X-axis direction and a Y-axis direction perpendicular to the X-axis direction, a control panel for setting a window with four sides along the X- or Y-axis direction on the display screen, and an edge detection section for integrating the lightness values of the pixels with respect to each pixel string arranged in the Y- or X-axis direction in the setup window, and detecting as an edge the position in the X- or Y-axis direction corresponding to the maximum value of the portion where the absolute value of the differential operation result in the X- or Y-axis direction, of the integration result is equal to or greater than a threshold value.