PURPOSE: To reduce the trouble for an instructing operation of a tracking start point by executing an input instruction of the tracking start point by inputting a tracking start point instructing segment for intersecting with a tracking linear image.

CONSTITUTION: When a linear image of image datg 13 subjected to tracking processing is displayed on a display screen of a display device 6, linear images 31, 32 of plural black picture element trains and an image in which an image area 33 of a black picture element area is displayed are displayed. Subsequently, tracking start point instructing segments P1-P2 for intersecting with the linear images 31, 32 on the display screen are inputted, an intersection in which the inputted tracking start point instructing segments P1-P2 and the linear images 31, 32 intersect with each other is discriminated, and by setting the discriminated intersection as tracking start points SP1, SP2, the tracking start point and the tracking start instruction are inputted. In such a way, it becomes unnecessary to execute an instruction of the tracking start point to the respective linear images, and trouble for an instructing operation of the tracking start point is reduced.