Usage of a ship handling simulator extend to many topics such as crew trains, evaluation of nautical instrument and research of human factor etc. In any case, it is necessary to observe operator’s behavior at bridge work. The procedures of performed operator’s task could be recorded by a monitoring video camera. However, it is difficult to estimate operator’s mental workload at each performance from the monitoring video. In this paper, practical technique is introduced for presuming navigator’s mental workload in ship handling sim- ulator by using data of navigator’s nasal temperature which is measured by thermal image video camera. And effectiveness of this technique is examined by some practical simulations.