A numerical study was made on the time-dependent ignition and subsequent transition to steady flame spread of a thin solid fuel on the floor, which is heated by an external radiation in a quiescent normal-gravity environment. The computational results indicate that little solid fuel is consumed before ignition occurs and the solid fuel surface reaches pyrolysis temperature when ignition occurs. Ignition is initiated at the middle of the sample to give a disk-shaped point flame at the instant of ignition. The point flame grows and immediately becomes an arch-shaped flame with two flame fronts at its bases. Then the arch-shaped flame breaks into two individual flames at the top, spreading in opposite directions. The flames spread rates in both directions were found to be faster than that of the vertical downward flame. The two spreading flames are in opposed mode because the directions between entrainment flow and flame spread are opposite. The entrainment velocity at the flame fronts, which are set up by the buoyancy, is about the order of several centimeters per second. The streamlines indicate that there are two types of flow; the ones parallel to the floor are the entrainment flow whereas the streamlines starting from the fuel surface are the result of blow off pyrolysis gases.