A wide and deep CCD photometry has been performed for three old open clusters, NGC 2420, NGC 1245, NGC 2506, using V and I filters. Our photometry covers 30\/x 30\/, 80\/x80\/, and 40\/x80\/ for NGC 2420, NGC 1245, and NGC 2506, respectively, and reaches to Mv≈10 for NGC 2420 and Mv≈9.5 for NGC 1245, NGC 2506, using BOAO SITe 2K CCD (NGC 2420) and CFH12K CCD (NGC 1245 and NGC 2506). Using isochrone fittings to the observed color-magnitude diagrams, we determined physical parameters (distance, age, metallicity and interstellar reddening) of the three clusters. We found clear evidences of mass segregation in the three clusters from the radial variation of LF and MF. We derived a lower limit of cluster mass using M-L relation of the main sequence stars: 1,068M⊙ (NGC 2420), 2,212M⊙ (NGC 1245), and 3,487M⊙ (NGC 2506). The half mass radii and the tidal radii derived from the cluster mass are 2.7pc. 6.1pc, 5.4pc, and 14pc, 18pc, and 21pc. respectively for NGC 2420, NGC 1245, and NGC 2506. The surface number density profiles and surface number density contour map strongly indicate the presence of tidal tails inside the tidal radius as well as the halo components. The contour maps of the surface number density clearly show that all the three clusters are elongated toward the Galactic center with the smallest ellipticity in NGC 2420 and largest ellipticity in NGC 2506.