According to one embodiment, a mask design method includes the following configuration. The method includes setting evaluation points on a circuit pattern created based on design data, setting a parameter that defines a shape of the mask pattern on the mask pattern corresponding to the circuit pattern and calculating an optical image intensity on the evaluation points set on the circuit pattern based on the mask pattern and the parameter. The method includes calculating an evaluation value relating to the optical image intensity by an objective function based on the optical image intensity and optimizing the parameter using a value of a partial differential for the parameter in the objective function to minimize the evaluation value calculated by the objective function.