The parameter determination method according to the present disclosure is a parameter determination method for determining a value of a parameter that is used for a simulation of determining gas transportability in a space inside a pore and that defines a boundary condition at an interface between a wall surface and gas or ions inside the pore, the method including determining the value of a parameter that reproduces a first concentration ratio indicating a ratio of the gas or ion concentration inside the pore to the gas or ion concentration outside the pore as the value of the parameter that defines the boundary condition.