A horizontal scanning rate correction apparatus for a cathode-ray tube, particularly of the beam index color cathode-ray tube type, includes a memory for storing correction values representing deviations of the horizontal scanning rate of the electron beam from a desired scanning rate; a reading circuit for reading the correction values from the memory; a correcting signal forming circuit for sensing fluctuation in the value of a high voltage supplied to an anode of the cathode-ray tube from a high voltage source and for producing a correcting signal in response thereto in accordance with the formula: .DELTA.V(t)=a(V.sub.H (t)-V.sub.HO)-b'x.sub.H .multidot.(d/dt.multidot.V.sub.H (t), where V.sub.H (t) is the instantaneous value of the high voltage, V.sub.HO is a constant value voltage, a and b are constants and X.sub.H corresponds to the amount of electron beam deflection; and a scanning rate modulating circuit supplied with the correction values read by the reading circuit and supplied with the correcting signal for substantially cancelling any deviation in the horizontal scanning rate from the desired rate.