  We examine the Eddington factor in an optically thick, relativistic flow
accelerating in the vertical direction. % When the gaseous flow is radiatively
accelerated and there is a velocity gradient, there also exists a density
gradient. The comoving observer sees radiation coming from a closed surface
where the optical depth measured from the observer is unity. Such a surface,
called a {\it one-tau photo-oval}, is elongated in the flow direction. In
general, the radiation intensity emitted by the photo-oval is non-uniform, and
the photo-oval surface has a relative velocity with respect to the position of
the comoving observer. Both effects introduce some degree of anisotropy in the
radiation field observed in the comoving frame. As a result, the radiation
field observed by the comoving observer becomes {\it anisotropic}, and the
Eddington factor must deviate from the usual value of 1/3. Thus, the
relativistic Eddington factor generally depends on the optical depth $\tau$ and
the velocity gradient $du/d\tau$, $u$ being the four velocity. % In the case of
a plane-parallel vertical flow, we obtain the shape of the photo-oval and
calculate the Eddington factor in the optically thick regime. We found that the
Eddington factor $f$ is well approximated by $f(\tau, \frac{du}{d\tau}) = {1/3}
\exp (\frac{1}{u} \frac{du}{d\tau}) $. % This relativistic variable Eddington
factor can be used in various relativistic radiatively-driven flows.
