In previous report, the interlock system of a pneumatic driving system have been studied and proposed based on the principle of safety. In this paper, we propose again that the interlock system is recognized high on the level of integrity as the safety-related parts of the international standard ISO13849. An accident may happen when failure of safety device may occur on the accidentally happened dangerous condition. It is a claim in this paper that in the safety system, failure of safety device

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... shall not be simply in the safe side, it is necessary to stop the system by shutting off the power supply. The interlock system satisfies the function on the condition of fail-safe and it is recognized high level of the evaluation from the point of view of risk-based international standards (ISO12100, ISO13849). And it is further expected an ideal safety-related systems which may not give any harmful effects and that this system is also expected to be recognized as the top-level category of qualification. Conclusion of this paper is that there are no contradiction between differently based validation according to the international standards and the principle of safety.