  In this series of papers, we model the formation and evolution of the
photoionized region and its observational signatures during massive star
formation. Here we focus on the early break out of the photoionized region into
the outflow cavity. Using results of 3-D magnetohydrodynamic-outflow
simulations and protostellar evolution calculations, we perform post-processing
radiative-transfer. The photoionized region first appears at a protostellar
mass of 10Msun in our fiducial model, and is confined to within 10-100AU by the
dense inner outflow, similar to some observed very small hypercompact HII
regions. Since the ionizing luminosity of the massive protostar increases
dramatically as Kelvin-Helmholz (KH) contraction proceeds, the photoionized
region breaks out to the entire outflow region in <10,000yr. Accordingly, the
radio free-free emission brightens significantly in this stage. In our fiducial
model, the radio luminosity at 10 GHz changes from 0.1 mJy kpc2 at m=11Msun to
100 mJy kpc2 at 16Msun, while the infrared luminosity increases by less than a
factor of two. The radio spectral index also changes in the break-out phase
from the optically thick value of 2 to the partially optically thin value of
0.6. Additionally, we demonstrate that short-timescale variation in free-free
flux would be induced by an accretion burst. The outflow density is enhanced in
the accretion burst phase, which leads to a smaller ionized region and weaker
free-free emission. The radio luminosity may decrease by one order of magnitude
during such bursts, while the infrared luminosity is much less affected, since
internal protostellar luminosity dominates over accretion luminosity after KH
contraction starts. Such variability may be observable on timescales as short
10-100 yr, if accretion bursts are driven by disk instabilities.
