An inevitable problem of the inertial measuring method is that the waveform is distorted by the high-pass filtering process for stabilizing the double integration of acceleration. We designed a method to overcome this problem, by combining characteristics of the inertial measuring method and mid-chord offset method. We call it an "inertial midchord offset method." By using a prototype track inspection device that applies the new method, we confirmed that it provides track irregularity waveforms similar to those obtained by the mid-chord offset method. Moreover, as part of that equipment, we developed a sensor to measure vertical and horizontal rail displacements at the same time.