PURPOSE: To improve the SN ratio, by extracting only the fudamental wave of index pulses and by adding the fundamental wave and the signal obtained by delaying it and by setting the matching impedance of the delay circuit to the characteristic impedance or less.

CONSTITUTION: The index signal from receiving tube 11 is received by detection sensor 8 and is amplified by band amplifier 12. The output is applied to band-pass filter 13, which extracts only index fundamental wave components, and band-pass filter 14 which extracts only main index fundamental wave components. Filters 13 and 14 hold band-pass characteristics considering fluctuation components of the fundamental frequency based on the horizontal deflection amplitude of the scanning beam and irregularity of linearity. Phase shifter 15 delays the main index signal, which is obtained from band-pass filter 14, by one period a least and adds the delayed signal and the output of filter 14, thereby stabilizing the main index signal.