PURPOSE: To obtain the transmittance modulation-type photomask which can produce optical parts having bidirectional rugged patterns by exposing once and the process for production of the optical parts by using this photomask.

CONSTITUTION: Apertures 6 and light shielding parts 7 are alternately arranged at prescribed intervals (a), (b) along first and second directions A, B. The width W1 along the first direction A of the light shielding part 7A existing between the apertures 6A and 6A changes along the second direction B in correspondence to the prescribed shape of the ruggedness 16 of a diffraction grating (optical part) 1. The two-dimensional diffraction grating 1 having the rugged patterns in the two directions A, B is produced by one time of exposing when this photomask 5 is used.