We report on an investigation of the voltage output from a magnetostrictive sensor for the measurement of elastic flexural waves in a cylindrical steel waveguide. Since the sensor performance is strongly influenced by the bias magnetic field, the bias field optimization is one of the most critical issues in the design of magnetostrictive sensors. For a magnetic system consisting of a yoke and an electromagnet, we formulate a method for optimizing yoke topology in order to maximize the sensor output. Both linear and nonlinear magnetization relations are considered in our analysis. For the verification of the performance of the proposed sensors, we conducted several experiments involving flexural waves to assess the performance of the optimized sensors, and we analyze their results here.