The effect of impregnated carbon dioxide (CO2) and pressure on dynamic viscoelastic responce of a commercial LDPE was investigated by a magnetic driven rotational rheometer equipped with a high-pressure chamber. It was found that i) the mode distribution of the viscoelastic relaxation is not sensitive to CO2 pressure realizing the superposition among moduli obtained at different pressures, that ii) the horizontal shift factor for the superposition reflecting the monomeric friction exponentially decreases with increase of the pressure, and that iii) the vertical shift factor relating to the entanglement number density is not sensitive to the pressure. From the observation it is suggested that CO2 and pressure mainly affect the monomeric friction and the effect on the entanglement dilation is not significant. The results are consistent with the dielectric measurement on polyisoprene (Matsumiya et al., Nihon Reoroji Gakkaishi, 35, 155-161, 2007).