A holding tool for holding the first and second side panels of a primary structure of an aircraft pylon during assembly thereof includes: a first portion configured to be removably secured to the first side panel, at least one second portion including at least one pair of first and second parts, positioned on either side of the second side panel and attracting one another by virtue of a magnetic field so as to secure the second portion to the second side panel, and at least one connection element connecting the first and second portions. Thus, during assembly of the primary structure, the second portion of the holding tool may be secured to the second side panel without the need for through-holes. The primary structure thus assembled does not comprise any through-holes in the side panels.