We report the pressure-induced insertion (PII) of gases in a zeolitic imidazolate framework (ZIF) at high pressure. ZIF-8 crystals were compressed in the presence of nonpolar CO2 and SF6 as well as polar CHClF2 (aka R22) up to 4.07, 0.81, and 2.06 GPa, respectively, in a diamond anvil cell, and their X-ray powder diffraction data were analyzed by Rietveld methods. ZIF-8 showed pressure-induced amorphization in SF6 but maintained its crystallinity in the presence of other gases. In general, the unit cell volume of ZIF-8 expanded via PII of CO2 or R22 up to ca. 0.5 GPa and contracted linearly after completion of the PII processes. The total volumes of the inserted guest molecules, which were estimated from the residual electron densities, indicated that the inserted guest molecules fully occupy the pores of ZIF-8. The relationship between the total occupied volumes by guests and applied pressures was best represented with an asymptotic logarithmic curve regardless of the type of molecules. These results reveal the universal gas-uptake behavior of ZIF-8 at high pressure, which requires only the penetration of molecules into the pores of the framework of ZIF-8.