  Radiatively-driven flow in a luminous disk is examined in the subrelativistic
regime of $(v/c)^1$, taking account of radiation transfer. The flow is assumed
to be vertical, and the gravity and gas pressure are ignored. When internal
heating is dropped, for a given optical depth and radiation pressure at the
flow base (disk ``inside''), where the flow speed is zero, the flow is
analytically solved under the appropriate boundary condition at the flow top
(disk ``surface''), where the optical depth is zero. The loaded mass and
terminal speed of the flow are both determined by the initial conditions; the
mass-loss rate increases as the initial radiation pressure increases, while the
flow terminal speed increases as the initial radiation pressure and the loaded
mass decrease. In particular, when heating is ignored, the radiative flux $F$
is constant, and the radiation pressure $P_0$ at the flow base with optical
depth $\tau_0$ is bound in the range of $2/3 < cP_0/F < 2/3 + \tau_0$. In this
case, in the limit of $cP_0/F = 2/3 + \tau_0$, the loaded mass diverges and the
flow terminal speed becomes zero, while, in the limit of $cP_0/F = 2/3$, the
loaded mass becomes zero and the terminal speed approaches $(3/8)c$, which is
the terminal speed above the luminous flat disk under an approximation of the
order of $(v/c)^1$. We also examine the case where heating exists, and find
that the flow properties are qualitatively similar to the case without heating.
