A phase shifting mask (PSM) for manufacturing a semiconductor device and a method of fabricating the same includes a transparent substrate, a main pattern formed on the transparent substrate and comprising a first phase shifting layer having a first optical transmittance greater than 0, and at least one assistant pattern formed on the transparent substrate proximal to the main pattern for phase-shifting by the same degree as the main pattern and having a second optical transmittance, which is less than the first optical transmittance.