center thereof the tears type mark is recognized as a tapered mark a combination of sampling values exhibiting a displacement pattern that has 3 u00b1 0 5 succeeded by 1 u00b1 0 5 is detected at sampling timings nos 3 and 4 from this fact the tapered mark is inferred consequently sampling values detected at sampling timings nos 5 to 9 are forcibly corrected to 0s and the sampling value detected at the sampling timing no 10 is forcibly corrected to 1 further when a transition pattern like the one shown in fig7 c is exhibited by a signal of a mark having a length of 6t sampling timing no 7 is the sampling timing at which the specific amplitude level value of 2 u00b1 0 5 inherent to a tears type mark is detected since the specific amplitude level value of 2 u00b1 0 5 exists in the second half of the transition pattern beyond the center thereof the tears type mark is recognized as a tapered mark a combination of sampling values exhibiting a displacement pattern that has 3 u00b1 0 5 succeeded by 1 u00b1 0 5 is detected at sampling timings nos 2 and 3 from this fact the tapered mark is inferred consequently sampling values detected at sampling timings nos 4 to 6 are forcibly corrected to 0s and the sampling value detected at the sampling timing no 7 is forcibly corrected to 1 processing proceeds to step s 15 of fig6 if processing termination is instructed the processing is terminated if the processing termination is not instructed processing returns to step s 1 on the other hand if a tears.