A storage medium includes a program which causes a computer to execute a method of calculating a light intensity distribution on a pupil plane of an illumination optical system. The method includes: determining an impulse response function of a projection optical system by performing Fourier transform on a pupil function of the projection optical system; setting a length to a second zero point of the impulse response function as a response length, extracting, from elements forming a target pattern, only elements inside am area within radius which is response length, and determining a function indicating the extracted pattern as an image function; and obtaining the light intensity distribution based on the pupil function, the determined impulse response function, and the determined image function.