A semiconductor device manufacturing method includes applying illumination light to a photomask, and projecting diffracted light components from the photomask via a projection optical system to form a photoresist pattern on a substrate. The photomask includes a plurality of opening patterns which are arranged on each of a plurality of parallel lines at regular second intervals in a second direction and which have regular first intervals in a first direction perpendicular to the second direction. The plurality of opening patterns arranged on the adjacent ones of the plurality of parallel lines are displaced from each other half the second interval in the second direction. Moreover, the dimensions of the plurality of opening patterns and the complex amplitude transmittance of nontransparent region in the photomask are set so that three of the diffracted light components passing through the pupil of the projection optical system have equal amplitude.