An inertial measuring method has a problem that its waveform is distorted by high-pass filter processing for stabilization of double integration of acceleration. We designed method to avoid this problem, by combining characteristics of the inertial measuring method and mid-chord offset method. We call it the "Inertial mid-chord offset method." By using tester that applies the new method, we confirmed that it provides track irregularity waveform similar to those obtained by the mid-chord offset method. Moreover, as part of that equipment, we developed a rail displacement sensor to measure vertical and horizontal displacements at the same time.