A system for modifying properties of an article of footwear is disclosed, wherein the article of footwear includes a removable foot-supporting member that includes a frame portion that defines an aperture, one of a plurality of insert portions that are received by the aperture, and a restraining member attached to an upper surface of the frame portion. The binding member is fixed to the tube and slidably supports the longitudinally extending member. An AMR element is located in a magnetic field in which the strength of the cross magnetic field in the spacing direction is within a predetermined range, and detects, as change in a resistance value, change in the cross magnetic field caused by an object to be detected. Therefore, the number of calculations for determining correlation values in a cell search is remarkably reduced.