Stress-ratio-type structural optimization algorithm for inhomogeneous elastic structures (trusses) is considered. The structures are loaded by a single system of static loads. It is demonstrated that the algorithm, known as a heuristic one, may be considered in some particular case as actually based on optimality conditions. The purpose of the paper is to reveal the case and conditions when the algorithm is monotonous, prove the monotonicity, and demonstrate convergence of the algorithm.