부산대학교 도서관

A trouble detecting unit in an optical security device intentionally produces a state which is equivalent to a state in which the security system is out of order so as to check whether an emergency stop signal is produced or not and judges that the security system is in order if the emergency stop signal is produced and that the security system is out of order if the emergency stop signal is not produced. This checking is made while the security system need not be in operation. In a press, for example, the checking is made while the press is being lifted from a bottom dead center. A state which is equivalent to the one in which the security system is out of order is produced by intercepting, one by one, signals to light emitting elements or signals from light receiving elements.