The concept of mental algorithm, a way of understanding the plant operator's thinking process for identifying malfunctions, has been proposed. According to the concept of the mental algorithm, the operator's thinking process includes many subtle decision–making affected by metacognition activity and the each subtle decision–making makes or chooses an operator's cognitive state (CS) and a behavioral strategy (BS). Most behavior as a result of subtle thinking is affected by performance shaping factors (PSFs), and the both the CS and the BS could be deteriorated. In this regard,a mental algorithm indicates the quality of problem–solving affected by PSFs,and could be applicable not only to the plant operator's thinking process but also to similar problem–solving. As an example of the problem-solving,this study dealt with the thinking for evacuation behavior. The participants moved from a start location to an evacuation center using a custom digital map on a personal digital assistance (PDA). As the results, we assumed that the thinking for evacuation could be represented as a mental algorithm. Further, the CS was categorized into three types and the four types of BS were extracted and the PSF items which lead to the deterioration of evacuation behavioral quality was estimated. Consequently,it was revealed that the concept of mental algorithm was applicable to evacuation behavioral assessment.
