A mask fabricating method includes dividing an outline of a first design layout for a target layer into plural segments, selecting interest segments to be biased in a direction of approaching an outline of a second design layout for a lower layer of the target layer, performing optical proximity correction for the target layer based on a first cost function given to each of normal segments and a second cost function given to each of the interest segments, and fabricating the mask corresponding to the first design layout updated based on a result of the optical proximity correction. The second cost function includes a model of a margin between each of the interest segments and the outline of the second design layout. Performing the optical proximity correction includes biasing each of the interest segments up to a boundary defined by the margin.