The experiments with regard to the effect of an eccentric phase angle of the pulley were made for a transmission error in a synchronous belt drive with an eccentric pulley in the quasi-static condition under an initial tension, and these experimental results were compared with the computed results. The eccentric phase angle of the pulley has a large effect on the transmission error. In order to maintain a small transmission error, the belt should be set up such that the eccentricities of the driving and driven pulleys cancel each other. When there is a meshing phase angle at the beginning of the meshing of the driven side, the transmission error having a period of one pitch of the pulley is decreased when this phase angle approaches zero, and the meshing phase angle exerts almost no influence on the transmission error having a period of one revolution of the driving pulley. Further, it was found that the amplitude of the transmission error having a period of one revolution of the driving pulley remains nearly constant regardless of the size of the belt width when the initial tension per unit width is the same.