The thinking faculty and operating ability of ship operators tend to be affected by their mental workload under the pressure from navigation environment. The suitable safety measures have to be taken to cope with the relation between stress degrees of operators and human errors based on the state-transition during operating man-machine systems such as a ship maneuvering system. This paper, on the subject of the state-transition and stress degrees of operators, consists of (1) the estimation of accident incident probability based on the fault tree analysis and (2) the quantitative analysis of the tensional stresses by measuring the fractal nature of heart rate variability under mental work-load during maneuvering several kinds of ships. Then these results are compared with the environmental stress model proposed by K. Inoue et al., and the specific property of tensional stress generated under severe environment may be clarified with a view of to prevent marine accidents.