The anomaly in binary diffusion coefficients has been reported by Nishiumi and co-workers in the near-critical region of pure CO2 solvent with the Taylor dispersion technique when the relatively large amount of solute as a tracer was injected. We have demonstrated that the mixture of the solute and CO2 solvent does not attain to the supercritical state at most axial positions of the diffusion column under their measurement conditions showing the anomaly.