The Kepler mission has revealed the prevalence of volatile-poor super-Earths and mini-Neptune in the proximity of host stars. Several post-formation processes have hitherto been proposed for explaining the origin of volatile inventory of those planets: a mass loss via a stellar XUV irradiation and Parker wind, degassing of accreted material, and in-situ accumulation of the disk gas. However, the compositional diversity between neighboring planets on adjacent orbits such as Kepler-36 and Kepler-11 systems is puzzling for the three processes. We consider the possibility of a collisional origin for the coexistence of volatile-poor super-Earths and mini-Neptunes in the proximity of host stars.We present the results of three-dimensional hydrodynamic simulations of giant impacts on a super-Earth with a H/He atmosphere. A high-speed collision can strip off most of the original H/He atmosphere, as we expected. A hot and inflated planet after the giant impact cools and contracts so slowly that a protracted state of the extended post-impact atmosphere enhances mass loss via both a Parker wind and subsequent hydrodynamic escape driven by a stellar XUV irradiation. We also found that a low-speed head-on collision results in the appearance of a positive-compositional gradient deep inside the planet which suppresses the efficiency of heat transport in the planetary interior, whereas a high-speed one can homogenize the refractory material above the core inside the planet.