  One of the candidates for Type Ia supernova progenitors, the recurrent nova
RS Ophiuchi underwent the sixth recorded outburst in February 2006, and for the
first time a complete light curve of supersoft X-ray has been obtained. It
shows the much earlier emergence and longer duration of a supersoft X-ray phase
than expected before. These characteristics can be naturally understood when a
significant amount of helium layer piles up beneath the hydrogen burning zone
during the outburst, suggesting that the white dwarf (WD) is effectively
growing up in mass. We have estimated the WD mass in RS Oph to be 1.35 \pm 0.01
M_sun and the growth rate of the WD mass to be at an average rate of about 1
\times 10^{-7} M_sun yr^{-1}. The white dwarf will probably reach the critical
mass for Type Ia explosion if the present accretion continues further for a few
to several times 10^{5} years.
