A planar insertion device and supporting structure for a planar insertion device for treating a synchrotron radiation beam includes a primary frame on which at least two secondary C-frames are mounted. An upper and a lower girders are mounted on the secondary C-frames forming a gap between girders and arranged substantially horizontally and parallel to each other and to the synchrotron radiation beam. Magnetic arrays rigidly mounted on the girders are facing each other and facing the gap between girders, with the synchrotron radiation beam passing between the magnetic arrays through the gap. The planar insertion device supporting structure prevents detrimental deformation reactions to variations of magnetic loadings with changes in the gap and subsequent geometrical misalignments.