Dielectric constant of TGS was measured during X-ray irradiation in both the para- and ferro-electric phases with and without an external dc bias field. Experimental results are collectively interpreted by dealing with the reciprocal dielectric constant. Irradiation effects on the dielectric constant are essentially independent of both the temperature of exposure and the application of dc field. The vaccine effect against an irradiation in the ferroelectric phase, appears in specimens previously irradiated in the paraelectric phase, is not a substantial but a seeming one. Taking the absorption of X-ray into account, we can estimate the density of defects formed by the irradiation.