In this study, we devise a technology to simultaneously inspect vehicles, bridges, and road surfaces from vibration data of traveling vehicles at the same time. To estimate the parameters of vehicles, bridges and road surfaces by convergent computation, the shape of the objective function is important because it affects not only
the accuracy but also the efficiency. As a result of numerical calculation, it was found that the objective function is always convex downward, even under the influence of measurement noises. This result suggests that the optimal solution can be searched by a simple method such as gradient descent.