A theoretical analysis of a transmission error with the condition of the contact state of the belt and the pulley teeth in the quasi-static condition under an initial tension was made for a situation in which a radial runout by an eccentric pulley occurs in a synchronous belt drive. The computed and the experimental results were compared. It was confirmed that the transmission error occurring with use of the eccentric pulley is mainly caused by periodic variance of the belt tension, and that it appears with a period of one pitch of the pulley and one revolution of the driving pulley. The tendencies of the experimental results and the computed ones coincided well. Further. it was found that there is an appropriate initial tension which lessens the transmission error having a period of one revolution of the driving pulley.