We measured an absolute ortho-D-2 concentration of gas samples taken from irradiated liquid D-2 samples as a function of the integrated radiation dose and the irradiation time by means of a frequency-resolved Raman spectroscopy to study the ortho-D-2 conversion mechanism under irradiation. The measurement was carried out by irradiating liquid ortho-D-2 at a temperature of 25 K with Bremsstrahlung photons, which were produced by bombarding tantalum with 33 MeV electrons, and whose absolute fluxes were determined experimentally by employing a foil activation method. The obtained ortho-D-2 concentration was found to decrease from 98% to 82% monotonically with increasing the radiation dose, irrespective of the electron-beam power. This fact clearly indicates the important role of a radiation-induced breakup effect of the ortho D-2 under the present experimental conditions of radiation dose and irradiation time. The obtained conversion rate is ten times faster than the evaluated value based on an existing model, requiring an alternate mechanism to enhance the conversion.