  Formation of massive stars by accretion requires a high accretion rate of >
10^-4 M_sun/yr to overcome the radiation pressure barrier of the forming stars.
Here, we study evolution of protostars accreting at such high rates, by solving
the structure of the central star and the inner accreting envelope
simultaneously. The protostellar evolution is followed starting from small
initial cores until their arrival at the stage of the Zero-Age Main Sequence
(ZAMS) stars. An emphasis is put on evolutionary features different from those
with a low accretion rate of 10^-5 M_sun/yr, which is presumed in the standard
scenario for low-mass star formation. With the high accretion rate of 10^-3
M_sun/yr, the protostellar radius becomes very large and exceeds 100 R_sun. It
is not until the stellar mass reaches 40 M_sun that hydrogen burning begins and
the protostar reaches the ZAMS phase, and this ZAMS arrival mass increases with
the accretion rate. At a very high accretion rate of > 3 x 10^-3 M_sun/yr, the
total luminosity of the protostar becomes so high that the resultant radiation
pressure inhibits the growth of the protostars under steady accretion before
reaching the ZAMS stage. Therefore, the evolution under the critical accretion
rate 3 x 10^-3 M_sun/yr gives the upper mass limit of possible
pre-main-sequence stars at 60 M_sun. The upper mass limit of MS stars is also
set by the radiation pressure onto the dusty envelope under the same accretion
rate at 250 M_sun. We also propose that the central source enshrouded in the
Orion KL/BN nebula has effective temperature and luminosity consistent with our
model, and is a possible candidate for such protostars growing under the high
accretion rate. (abridged)
