  We present 250~GHz continuum and H29alpha line data toward W49N:A2, a
hypercompact HII region ionized by an O9 star. The data obtained with ALMA at a
resolution of ~0"05 (600 au) confirmed the presence of an ionized ring with a
radius of~700 au inclined by ~50degree (0degree for pole-on). It has a width of
~1000 au and is relatively flat with a scale height of less than several
hundred au. The tilted ring, or the apparent ellipse, has a prominent velocity
difference between its NW and SE ridges along the minor axis, suggesting that
it is expanding in the equatorial plane at a velocity of 13.2 km/s. The ring
also shows a hint of rotation at 2.7 km/s, which is significantly (2.5sigma)
smaller than the Kepler velocity of 5.2 km/s at its radius around the 20 M$_sun
star. This can be interpreted that the ring gas has been transported from the
radius of ~170 au by conserving its original specific angular momentum that it
had there. The ionized ring may thus be a remnant of the accretion disk that
fed the O9 star, whose radiation or magnetic activities became so strong that
the disk accretion was reversed due to the intense thermal or
magneto-hydrodynamic pressure around the star. The data has revealed a rare
example of how a massive star terminates its accretion at the end of its
formation, transforming a hypercompact HII region into an ultracompact HII
region.
