A method of forming patterns wherein a first exposure is performed using a first alternating phase shift mask, and a second exposure is performed using a second alternating phase shift mask. Phase shift regions and non-phase shift regions of the second mask are made to correspond respectively to non-phase shift regions and phase shift regions of the first mask. Consequently, light transmitted through phase shift regions of the first mask during the first exposure, is transmitted through second non-phase shift regions of the second mask during the second exposure, so that weak light intensity is compensated for. Intensities of light passed through phase shift regions and non-phase shift regions are thus the same. Therefore, the &Dgr;CD phenomenon and the inversion phenomenon of critical dimensions between phase shift regions and non-phase shift regions with respect to different focuses, may be prevented.