Dielectric measurement was conducted for cis-polyisoprene (PIP) in carbon dioxide (CO2) at high pressures. CO2 is dissolved in PIP more with increasing pressure, and this dissolution is expected to affect the chain dynamics. In our experimental window, the PIP chains having type-A dipoles parallel along their backbone exhibited the dielectric dispersion reflecting their global motion. This dispersion shifted to higher frequencies with increasing CO2 pressure. Time - CO2 pressure superposition was found to hold well, which suggests that the global motion of the chain is accelerated by CO 2 without any heterogeneity in space and along the chain backbone. The vertical shift factor (intensity shift factor) for this superposition gave the CO 2 concentration in PIP. The plasticizing capability of CO2 evaluated from the horizontal shift factor (frequency shift factor) was found to be a little larger than that of tetradecane in the same range of concentration. No peculiarity was found for plasticizing ability of CO2 even in the supercritical state (at a pressure of 10 MPa).