Josephson critical current I-c between a single crystal CePt3Si and Al has been measured for the junctions on the CePt3Si surface perpendicular to the a and c axes. In the magnetic-field dependence of I-c a dominant maximum peak at zero field has been observed only for the junction on the a-axis face thus far, indicating that the Josephson effect is allowed at least for current flow parallel to the a axis. This result places a restriction on the representations of the order parameter in the noncentrosymmetric superconductor CePt3Si.