A gas diffusion simulation method for simulating diffusion of a gas in a porous material having many pores, the method includes: calculating, in the pores, a Knudsen diffusion coefficient based on the mean square displacement of first gas particles in spaces surrounded by wall surfaces and a Knudsen diffusion term using the Knudsen diffusion coefficient, calculating an interdiffusion term using an interdiffusion coefficient between the first gas particles and second gas particles different therefrom, and performing simulation of the gas diffusion of the first gas particles by using a diffusion equation of the first gas particles represented by the sum of the Knudsen diffusion term and the interdiffusion term.