Experiments on a transmission error having a period of one revolution of the belt in a polyurethane synchronous belt drive were performed in the quasi-static condition under an initial tension. The experimental results were compared with the computed results which used the results of measurement for the production error in the belt. It was confirmed that the transmission error is mainly caused by a single pitch error and the change of the product of the sectional area of the load carrying cords and the modules of elasticity of the belt, SE. In the case of the belt used in this study, the change of SE had the most influence on the transmission error. As the tendencies of the experimental results and the computed ones coincided well, the methods of measurement for the single pitch error and SE, and the methods for arrangement of these values used in order to compute the transmission error were right.