  In phenomenological models with D-branes, there are in general open-string
massless scalar fields, in addition to closed-string massless moduli fields
corresponding to the compactification. It is interesting to focus on the fate
of such scalar fields in models with broken supersymmetry, because no symmetry
forbids their masses. The one-loop effect may give non-zero masses to them, and
in some cases mass squared may become negative, which means the radiative gauge
symmetry breaking. In this article we investigate and propose a simple method
for calculating the one-loop corrections using the boundary state formalism.
There are two categories of massless open-string scalar fields. One consists
the gauge potential fields corresponding to compactified directions, which can
be understood as scalar fields in uncompactified space-time (related with
Wilson line degrees of freedom). The other consists "gauge potential fields"
corresponding to transverse directions of D-brane, which emerge as scalar
fields in D-brane world-volume (related with brane moduli fields). The D-brane
boundary states with constant backgrounds of these scalar fields are
constructed, and one-loop scalar masses are calculated in the closed string
picture. Explicit calculations are given in the following four concrete models:
one D25-brane with a circle compactification in bosonic string theory, one
D9-brane with a circle compactification in superstring theory, D3-branes at a
supersymmetric C^3/Z_3 orbifold singularity, and a model of brane supersymmetry
breaking with D3-branes and anti-D7-branes at a supersymmetric C^3/Z_3 orbifold
singularity. We show that the sign of the mass squared has a strong correlation
with the sign of the related open-string one-loop vacuum amplitude.
