PURPOSE: To present a yield simulation method and a simulator which can perform simulation at a higher speed by preliminarily giving the classification of trouble due to one defect and its occurrence probability.

CONSTITUTION: This yield simulator based on the Monte Carlo method which determines the number of defects in a system as the object by random numbers is provided with a trouble classification table 1 where classifications of trouble due to defects and their occurrence probabilities are held, a determining means 3 which determines the classification of trouble due to each defect by random numbers, and an arithmetic means 5 which obtains the yield of the object system based on determined classifications of trouble.