  We present 1500 cycles of hydrogen shell flashes on a 1.38 M_sun white dwarf
(WD) for a mass accretion rate of 1.6 x 10^{-7} M_sun yr^{-1}, the mass
ejection of which is calculated consistently with the optically thick winds.
This model mimics the one-year-recurrence-period nova M31N 2008-12a. Through
these hydrogen flashes a helium ash layer grows in mass and eventually triggers
a helium nova outburst. Each hydrogen flash is almost identical and there is no
precursor for the forthcoming He flash either in the outburst or in the
quiescent until the next He flash suddenly occurs. Thus, M31N 2008-12a is a
promising candidate of He novae, outbursting in any time within a millennium
years. The prompt X-ray flash of He nova lasts as short as 15 min with the
X-ray luminosity being about a half of the Eddington luminosity, making the
observation difficult. In the very early phase of a He flash, the uppermost
H-rich layer is convectively mixed into the deep interior and most of hydrogen
is consumed by nuclear burning. In comparison with hydrogen shell flashes of
M31N 2008-12a, we expect the forthcoming He nova with a very short prompt X-ray
flash (15 min), a very bright optical/NIR peak (~3.5 mag brighter than M31N
2008-12a), a much longer nova duration (>2 years), and a longer supersoft X-ray
source phase (40-50 days or more).
