Recent helical synchronous belt drives have been confirmed to be effective for reducing noise and transmission error per single pitch of the pulley, in comparison with conventional synchronous belt drives in which the tooth trace is straight and parallel to the axis of rotation. However, in a helical synchronous belt, the helix angle produces an axial force when the belt tooth load acts in the direction normal to the tooth trace. In the present study, the transmission error in a helical synchronous belt drive is investigated theoretically and experimentally under the quasi-static condition and transmitted torque when the pulley speed ratio is 1 : 1. The computed transmission error agrees well with the experimental results, thereby confirming the applicability of the proposed theoretical analysis for the transmission error in bidirectional operation. This transmission error was found to be generated by a change in the belt tension under transmitted torque, by a change in the state of contact between the tooth flanks, and by the difference in axial belt movements between the two pulleys. In addition, the transmission error is reduced when the pitch difference of the driving side is smaller than that of the driven side.