PURPOSE: To achieve higher accracy in the correction of distance of a magnetic force data and moreover, enable measurement of a surface magnetic force in real time by measuring the interval of the surface and the tip of a magnetic probe directly in an accurate manner when the surface magnetic force of a roll-shaped magnet is measured with a magnetic probe.

CONSTITUTION: A distance measuring means 3 which comprises a magnetic probe 2 fixed with a fine gap G between a surface 4 and the tip 5, a projector 6 for generating detection light L with a diameter of a beam larger than the interval and a photodetector 7 for detecting the detection light is provided near the surface of a roll-shaped magnet 1. The projector 6 and the photodetector 7 are arranged sandwiching the gap in such a manner that optical axes thereof are set parallel in a tangential direction of the roll-shaped magnet 1 while the interval is positioned within the diameter of the beam of the detection light. Thus, a magnetic force data measured with the magnetic probe 2 is corrected by an interval data measured utilizing the fact that the quantity of the detection light passing through the photodetector varies with changes in the interval by a distance measuring means 3.