  The recurrent nova RS Ophiuchi, one of the candidates for Type Ia supernova
progenitors, underwent the sixth recorded outburst in February 2006. We report
a complete light curve of supersoft X-ray that is obtained for the first time.
A numerical table of X-ray data is provided. The supersoft X-ray flux emerges
about 30 days after the optical peak and continues until about 85 days when the
optical flux shows the final decline. Such a long duration of supersoft X-ray
phase can be naturally understood by our model in which a significant amount of
helium layer piles up beneath the hydrogen burning zone during the outburst,
suggesting that the white dwarf mass is effectively growing up. We have
estimated the white dwarf mass in RS Oph to be 1.35 \pm 0.01 M_\sun and its
growth rate to be about (0.5-1) \times 10^{-7} M_\sun yr^{-1} in average.
