Numerical simulation is carried out for solving a transient flowfield of a gas mixture consisting mainly of carbon dioxide in an expansion tube. The Navier-Stokes equations in the axi-symmetric form with a thermochemical nonequilibrium gas assumption are integrated numerically to examine flows in the shock tube as well as in the acceleration tube sections. Calculated results show that the basic structure of unsteady flowfield in the carbon dioxide gas is quite similar to that found in our previous study for air. Shock velocities of 4.4 and 7.5 km/s are attained in the shock tube and acceleration tube sections, respectively. It is found that the contact surface ahead of the driver gas is perturbed significantly when it passes through the nozzle at the entrance of the shock tube section. It is further deformed by the interaction with the boundary layer in the acceleration tube. The contact surface behind the transmitted shock wave in the acceleration tube is also perturbed considerably due to the interaction with the boundary layer. The estimated test time becomes about 10 μs which is about half of that for air.