To obtain the simulator that evaluates frequency of incidence of a low incidence probability in a system whose state is transited in a probable way at a high speed by providing each of simulation model setting means, initial state setting means, calculation of transition probability and calculation of incidence probability means.

An input section 49 receives various information. An environment setting section 50 sets various parameters based on characteristic information of the system received via the input section 49, models the network system being an evaluation object to set a proper traffic model. Furthermore, when information required to set an initial state and an intermediate state of the system is entered, a simulation environment corresponding to each state is set. An evaluation section 51 is a simulation engine and sends the information obtained as a result to a processing section 53. An output section 54 outputs a statistic variable obtained by the processing section 53 by using a CRT or a printer or the like.