Noncentrosymmetric conductors are an interesting material platform, with rich spintronic functionalities(1,2) and exotic superconducting properties(3,4) typically produced in polar systems with Rashba-type spin-orbit interactions(5). Polar conductors should also exhibit inherent nonreciprocal transport(6-8), in which the rightward and leftward currents differ from each other. But such a rectification is diffcult to achieve in bulk materials because, unlike the translationally asymmetric p-n junctions, bulk materials are translationally symmetric, making this phenomenon highly nontrivial. Here we report a bulk rectification effect in a three-dimensional Rashba-type polar semiconductor BiTeBr. Experimentally observed nonreciprocal electric signals are quantitatively explained by theoretical calculations based on the Boltzmann equation considering the giant Rashba spin-orbit coupling. The present result offers a microscopic understanding of the bulk rectification effect intrinsic to polar conductors as well as a simple electrical means to estimate the spin-orbit parameter in a variety of noncentrosymmetric systems.