The problem of RMS error determination in ionospheric parameters measured by the method of vertical HF soundings is considered here. This problem is solved for the case of topside ionospheric soundings when the height distribution of electron density is monotonic. An analytical expression is obtained which relates the variance of deduced parameters to the correlation function of errors in measured characteristics of sounding signals. The computing simulation provides for errors in real height determination, in reconstruction of vertical velocities of plasma motion, and in effective electron collision frequencies for the measurements of virtual heights, Doppler frequency shift, and radio wave absorption, respectively. Their dependencies on signal polarization, working frequency range, and geomagnetic latitude of observation point are determined. The obtained results are useful for planning plasma diagnostics experiments using HF sounding technique.