We present internal proper motion measurements of more than five hundred positions of NGC 2392, based on images acquired with the WFPC2 on bord of the Hubble Space Telescope for two epochs separated by 8.805 yr. We measured the amount of motion of local structures in the nebula determining their relative shift during that interval. We compared two different methods to determine the proper motions. Using those proper motions and the radial velocity taken from high-resolution spectroscopic observations, we were able to estimate a distance of 1050 pc.