Influence of the error in the Stokes drag coefficient and that of unsteadiness on the correction method for velocity measured by PIV and other techniques using particle tracers. Non-dimensional formulations of the correction method for general drag coefficients were derived. The corrected velocity using the Stokes drag coefficient was overestimated when the Stokes drag coefficient was smaller than the real drag coefficient, and vice versa. Non-dimensional correction equation for unsteady flow fields was also derived. In the flow fields except the region close to the shock wave, the influence of the unsteady correction terms was small and could be neglected if the Stokes number is sufficiently small. Although the influence of the error of the Stokes drag coefficient and that of the unsteadiness on a corrected velocity was not negligible in the region close to the shock wave, the correction method based on the Stokes drag coefficient for the steady flow was effective to obtain the more accurate velocity in the velocimetry for the unsteady flow investigated in the present study.