particularly an electron beam 4 constituting a red or green or blue electron beam generated by a drive signal s10 constituting a red or green or blue drive signal is caused to scan the index pattern 3 fig3 a and detected signals s11 thereby obtained fig3 b1 e1 are subjected to the slicing process in the waveform shaping circuit 13 the rising times of the signals with shaped waveform s12 fig3 b2 e2 obtained as a result of the aforementioned process deviate from normal values as a result of noise mixed in with the detected signals s11 the ramp signals s14 including such deviations fig4 and 5 are obtained in the ramp signal generator 14 and these are received by the error data retaining circuit 16 as the digital ramp signal data s19 the ramp signal data s19 received by the error data retaining circuit 16 are as described with reference to fig3 5 in the foregoing adapted to be obtained on the basis of the integrated values according to the integral controlling signals s13 formed as the result of sequential scanning by the electron beam 4 along a plurality of scanning lines for example four scanning lines scn1 scn4 for each index pattern 3 therefore even though noise is inevitably mixed.