A semiconductor device simulation method includes the step of storing, in a storage unit, a surface potential and threshold voltage obtained by computation, the step of computing thermal drain noise on the basis of the data of the surface potential and thermal drain noise stored in the storage unit, and the step of determining whether or not to reduce thermal drain noise, and reflecting the computation result in simulation of the model when it is determined that thermal drain noise is to be reduced. A drain current I.sub.ds of a MOSFET is calculated and substituted into a relational expression for a drain current noise spectrum density obtained from a Nyquist theorem equation, thereby calculating a thermal drain noise coefficient .gamma. of the MOSFET by substituting the current I.sub.ds into a relational expression for a thermal drain noise spectrum density which is obtained from the Nyquist logical equation.