In order to analyze exhaustively affects of failures in small embedded systems, we proposed SASTD (Safety Analysis method based on State Transition Diagram). SASTD assumes that system specifications of an embedded system have been modeled as only one state transition diagram. Therefore it is difficult that we analyze exhaustively the state transition diagram including many states and state transitions by using SASTD. In many cases, a hierarchical state transition diagram is used to reduce the number of states in a state transition diagram. In this paper, we propose SAHSTD (Safety Analysis method based on Hierarchical State Transition Diagram). We applied both SASTD and SAHSTD to the system specification of an electric boiling pot, and compared the results of them. Consequently, we confirmed that all deviations derived by SASTD could be also derived by SAHSTD. Since several system states with same characteristics were integrated to hierarchical system states, the number of derived deviations that analyzer must decide its severity could be reduced from 92 to 61 in the analysis for system states, and from 284 to 134 in the analysis for system state transitions, respectively.%小規模な組込みシステムにおける故障の影響をより網羅的に分析することを目的に，1つの状態遷移図に基づく安全分析手法SASTD（Safety Analysis method based on State Transition Diagram）を提案した．大規模な組込みシステムでは，1つの状態遷移図に含まれる状態数が多くなるため，SASTDで網羅的に分析するのが困難であるという問題がある．1つの状態遷移図に含まれる状態数を減らすためには，小規模な状態遷移図に分割した階層型状態遷移図を利用するのが有効である．本論文では，SASTDを拡張した，階層型状態遷移図に基づく安全分析手法SAHSTD（Safbty Analysis method based on Hierarchical State Transition Diagram）を提案する．SAHSTDを話題沸騰ポットのシステム仕様に対して適用した結果，SASTDと同一の逸脱を分析できることを確認した．さらに，同一の性質をもつ状態を階層的に整理することで，分析者が本質的に分析すべき逸脱数が，SASTDに比べて，状態に対する分析では92個から61個に，状態遷移に対する分析では284個から134個に削減できることが明らかになった．
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