PURPOSE: To obtain a simulation system by which a simulation can be continuously executed without stopping it even at the time of the generation of an unsettled value, and the executing time of the simulation can be reduced.

CONSTITUTION: This device is equipped with an indefinite value detecting part 4 which detects the indefinite value generating time and net, indefinite value cause retrieving part 5 which retrieves the cause of the generation of the indefinite value, indefinite value net logical value fixing part 6 which fixes a logical value '0' or '1' to the indefinite value generating net so that the result of a logical simulation can be coincident with an expected value, and logical simulation result/message outputting part 9 which outputs the result of the logical simulation, the indefinite value generating time and the name of the indefinite value generating net, fixed logical value, and the cause of the generation of the indefinite value or the like. Thus, the reduction of the executing time of the simulation, and the improvement of a debug efficiency due to the output of the cause of the generation of the indefinite value can be attained.