The rotational transmission error of the strain wave gearing, known as Harmonic Drive, is the main cause of the tortional vibration. In this study, the transmission errors of short-cut-type strain wave gearing, that has continuously contact tooth profile, were measured using high precision test device under the no load condition, loaded condition and dynamic condition. The main results are as follows. The rotational transmission error of the strain wave gearing is not only composed of components due to pitch errors of the circular spline and flexspline tooth, but also component due to thickness variation of the outer ring of the wave generator. The thickness variation of the outer ring causes iess repeatability of the rotational transmission error for each output rotation. Difference of pitch error between right and left side tooth surfaces is the cause to change the tortional transmission error in several conditions of loads. Effects of thickness variation on the outer ring become larger in dynamic condition.