<div class="htmlview paragraph">The transmission error of meshing gears is usually measured under the condition where a light load torque is applied to the gear rotating at a low speed. In practice, however, gears are integrated into various systems in diverse manners and the rotating speed and the load torque changes at a wide range. An accurate measurement and evaluation of the gear transmission error at such the operating condition are strongly required.</div> <div class="htmlview paragraph">We have constricted a measuring instrument with which the transmission error of gears is measured at various rotating speed and load torque and investigated how to measure the gear transmission error with precision. The torsional vibration modes of the power transmission system in the measuring instrument were analyzed for the investigation of error generation and the use of the frequency range between the first and the second mode was determined to avoid the effect of the resonant vibration of the power transmission system.</div> <div class="htmlview paragraph">This paper describes the outline of the measuring instrument, the reason of errors in the measurement, the method to minimize the error and the performance of the improved measuring instrument with some examples.</div>