PURPOSE: To easily and exactly form a guide of the specified width in small man-hours by moving a quantitative dispersion machine along a magnetic guide forming route, which machine has a hopper filled with granular body magnetic guide material.

CONSTITUTION: A quantitative dispersion machine comprises a dispersion machine 10 provided connected to the opening of the lower end of a hopper 9 storing powder and granular body material 8 therein and a discharge guide 11 provided under the dispersion machine 10. In the next step, the hopper 9, dispersion machine 10 and discharge guide 11 fixed on a truck capable of moving with four wheels 14 on a construction face 12. A continuous magnetic guide 13 is formed with the quantitative dispersion machine moving along a magnetic guide forming route on a magnetic guide construction face to disperse the powder and granular body material 8 on the construction surface so that a magnetic guide B becomes 5(mm)≤B≤1000(mm) at a speed A of 0(m/min)<A≤500(m/min).