An image processor for performing trace processing for tracing the line image has an image memory for storing image data, a data memory for storing graphic data, a display unit for displaying the image data and the graphic data, an input unit for entering trace designation, a semi-automatic image trace processing section for performing the trace processing for tracing the line image of the image data in a semi-automatic way, and a tracing-rule description processing section for designating a trace method for tracing the line image. The line width of the line image is detected by designating a point in the vicinity of a pixel line as a start point for detecting the line width of the line image as an object of tracing the image data, retrieving a change point for changing pixels on a contour of the line image from the start point, and detecting the line width by detecting the number of the pixels in a continuous pixel line arranged in the line-width direction of the line image from the change point.