A unified fuel processing reactor for a solid oxide fuel cell can reform hydrocarbon-based fuel into hydrogen-rich gas, remove a sulfur component, and convert non-converted fuel and a low carbon (C2˜C5) hydrocarbon compound into hydrogen and methane in a single reactor. The reactor comprises a primary-reformer which reforms a hydrocarbon-base fuel and generates hydrogen-rich reformed gas, a desulfurizer which removes a sulfur component from the reformed gas, and a post-reformer which selectively decomposes a low carbon (C2˜C5) hydrocarbon in the desulfurized reformed gas into hydrogen and methane. The primary-reformer, desulfurizer and post-reformer are in the unified reactor and isolated, except for a fluid passage, from each other by internal partition walls. The primary-reformer is disposed at a center portion of the reactor. The post-reformer and the desulfurizer are concentrically disposed outside of the primary-reformer.