  We analytically investigate the formation of an HII region in the accreting
envelope of a newborn star. Special care is taken to examine the role of
ionizing radiation force. This effect modifies velocity and density
distributions and thereby affects the expansion of the HII region. As a result,
the upper limit of the stellar mass imposed by the growth of an HII region
around a forming star is increased by a larger factor than the previous
estimate. In particular, for a star forming out of metal-free gas, this
mechanism does not impose a firm upper limit on its mass.
