Defect probability calculating method and semiconductor device manufacturing method

A defect probability calculating method includes assuming a plurality of process conditions containing process variations caused in a process of forming a pattern on a substrate based on a design pattern, acquiring appearance probabilities of the respective process conditions, performing process simulation to predict a pattern to be formed on a substrate based on the design pattern for each of the process conditions, determining whether the pattern predicted by performing the process simulation satisfies preset criteria for each of the process conditions, and acquiring first probability by adding together appearance probabilities of the process conditions used for process simulation of patterns which are determined not to satisfy the preset criteria.