In the oxide superconductor with layered structure, the superconducting currents flow in the Cu–O2 plane that is parallel to the a–b axis. The critical current Ic of Bi2Sr2CaCu2Ox (Bi-2212) single crystals measured by a four-probe method did not increase linearly with increasing the thickness of the crystal. The saturated Ic value of the crystals was about 2.8A. The results meant that the current flow in the a–b plane is different from that in c-axis in the oxide superconductor. In the Cu–O2 planes that are parallel to the a–b plane, the superconducting currents flow in a conventional conduction system, whereas between the Cu–O2 planes that are perpendicular to the a–b plane, they flow by tunneling mechanism. We assumed the current path in a single crystal, and calculated the Ic by the superconducting current simulation. The results indicated that the superconducting current between the Cu–O2 planes decreased by 2.5% in every 1μm.