Synchronous belt drives are commonly used in conjunction with an idler on the back face of the belt. However, thickness errors between the belt pitch line and back face of the belt, if present, will result in a change in belt tension on the span, and are considered to affect transmission error. In the present study, the transmission error in asynchronous belt drive with an idler under no load was investigated both theoretically using a belt of known thickness error. The computed transmission error agrees well with the experimental data thereby verifying the applicability of the analysis method. In addition, a transmission error was mainly generated by the change in length of the belt pitch line due to the thickness error of the belt. It is shown that the transmission error due to the belt thickness error can be removed by using an automatic tensioner.