According to one embodiment, a method is disclosed for determining position of an auxiliary pattern on a photomask. The method can include generating a first set for each of three or more imaging positions of an exposure optical system. The method can include generating a second set for each of the three or more imaging positions by inverse Fourier transforming each of the first set. The method can include calculating a second order differential with respect to the imaging position of an index indicating amplitude of light belonging to the second set. In addition, the method can include extracting a position where the second order differential assumes an extremal value on an imaging plane of the exposure optical system. At least part of positions on the photomask each corresponding to the position assuming the extremal value on the imaging plane is used as a formation position of the auxiliary pattern.