The present invention provides an optical disk 9 with a duplication preventing function, reproducing
system 47 for the optical disk 9, optical disk reproducing method, method of fabricating an optical disk original
record, and illegal program operation stopping method. An area in which a CP
signal 24 is recorded is made after a
signal having a constant pattern in the optical disk, the CP
signal 24 being produced such that the center of an
eye pattern is shifted from the center of the amplitude. In the reproducing
system, CP signal arrangement information 25 or a CP preceding signal 26 is detected, before an offset
voltage is applied to a threshold value to be used in demodulating an RF signal into a
digital signal so that the threshold value varies, thus normally reproducing the signal in which center of the eye patter is shifted from the center of the amplitude. The reproducing
system has a function whereby the
reproduction is stopped when a predetermined offset signal is not reproduced.