When the data of a mask pattern of a phase shift mask is to be made, the pattern data is separated into a real pattern data layer having the data of real patterns and a phase shift pattern data layer having the data of phase shift patterns. After this, it is verified whether or not the mask pattern satisfies the regulation of the gap of in-phase patterns, in which lights having transmitted through patterns adjacent to each other are in phase. It is also verified whether or not the mask pattern satisfies the regulation of the gap of out-of-phase patterns, in which lights having transmitted through patterns adjacent to each other are out of phase.