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This paper proposes a nevel approach supporting evaluation of software Safety Integrity Level (SIL) which is a relative target level of risk-reduction provided by a safety function using failure frequency. Software safety on embedded systems has become an important software engineering challenge, since the embedded system is closely used to human life and the software directly controls most operations of the embedded system. To address this challenge, several techniques have been proposed to analyze and evaluate the software safety. However, although these analysis and evaluation techniques have been proposed, still experts need to qualitatively evaluate the software SIL, even if the representative international standard for safety, IEC 61508, deals with software SIL evaluation, since existing standards enumerate only recommended software techniques for evaluating software SIL. Therefore, certification of software SIL highly depends on a third-party consulting company and this high dependency makes additional cost. Hence, our approach can be used without the dependency before formal certification of software SIL. Our approach identifies possible failures on embedded systems and calculates a probability of failure frequency using Markov process. Using the probability, we define quantitative measures to evaluate software SIL. We also conducted a case study using open data to evaluate our approach. The result of case study demonstrates that our approach can be a reasonable method to support quantitative evaluation of software SIL without expert's knowledge.