one of the syndromes s py and s qy is not zero for the y direction subblock in question it is deemed that the two data words each having the bit 34 1 34 error flag are actually in error the decision step 8 12 is thus followed by the step 8 14 to correct these errors on the basis of the syndromes s py and s qy in accordance with equations 9 and 10 with the error locations i and j represented by the particular two words in the procedure described with reference to fig3 the flag bits for all the data and error check words of the subblock are neither reset to 34 0 34 s nor set to 34 1 34 s by the imaginary step 3 15 and are to be set or reset depending upon the results of the subsequent x direction error detection procedure c of the flowchart of fig2 if it happens that the subblock in question includes errors which have not been detected by the initial x direction error detection procedure a then false correction is made by the step 8 14 for the two actually correct words and as a consequence the particular subblock will have three erred words including the two falsely corrected words thus making it impossible to correct the initial erred word in the y direction error correction procedure b shown in fig6 the flag bits of all the words in the subblock are either set to 34 1 34 s or neither set to 34 1 34 s nor reset to 34 0 34 s depending upon.