The results of in situ measurements of supercooling of the interface during the growth of Bi4Ge3O12 single crystal from melt are presented. Supercooling was determined by measuring the intensity of intrinsic thermal radiation from the interface, which was recorded with an optical pyrometer through the growing crystal. The dependence of growth rate on supercooling was determined from the experimental data on supercooling versus the cooling rate for a thin layer of melt. The features of the determination of supercooling during the formation of macrosteps on the facet are considered. Experimental data are obtained concerning the dependence of growth rate on supercooling for crystal growth in the [211] direction.