A new helical synchronous belt has been developed to reduce noise. However, in addition to the noise characteristics, the transmission characteristics must be considered. The present study examines experimentally the transmission error in a helical synchronous belt drive caused by side tracking of the belt due to the helix angle of the tooth trace. Experiments were performed under quasi-static conditions and no load. Experimental results suggested that the transmission error that occurs over the course of one belt revolution was caused by the change in the belt pitch when belt zigzagging was limited by optimization of the flanged pulley because of the axial force of the belt. These results are in good agreement with computed results that considered the change in the belt pitch. Thus, the side face of the belt should be processed appropriately, because belt zigzagging is introduced under normal conditions when the cylindrical belt has been cut to a constant width. When the　installation tension is decreased and the distance between the driving and the driven shafts is shortened, this transmission error can be reduced.