It has been known that oxo amide crystals made up of achiral molecules can be photochemically converted into optically active beta-lactams by irradiation with a high pressure mercury lamp. Although the oxo amide crystal belongs to an enantiomorphic class D-2, its optical activity could not be detected so far. We successfully measured the components of the gyration tensor of the oxo amide crystal by using the HAUP (high accuracy universal polarimeter) method : g(11) = -3.5 x 10(-4) (-79 degrees/mm), g(22) = -1.4 x 10(-4) (-32 degrees/mm), g(33) = -3.0 x 10(-4) (-68 degrees/mm) at 305 K, where the optical rotatory powers are indicated in parentheses. It has been proved by direct experiments that the optical activity of the reactant and the product in this reaction system remains unchanged over the photocyclization process.