The thermal-birefringence-induced depolarization in terbium gallium garnet (TGG) ceramics has been investigated experimentally. The depolarization ratio of 6.1×10(-4) has been observed at the maximum input power of 117 W cw, which corresponds to a normalized laser power of p=0.14. As predicted by the previously proposed theory, the amount of depolarization ratio and its slope with respect to the laser power of the ceramic TGG was approximately the same as that previously reported for high-quality-cut <111> single crystal.