  We present the results of multi-epoch very long baseline interferometry
(VLBI) water (H2O) maser observations carried out with the VLBI Exploration of
Radio Astrometry (VERA) toward the Cepheus A HW3d object. We measured for the
first time relative proper motions of the H2O maser features, whose
spatio-kinematics traces a compact bipolar outflow. This outflow looks highly
collimated and expanding through ~ 280 AU (400 mas) at a mean velocity of ~ 21
km/s (~ 6 mas/yr) without taking into account the turbulent central maser
cluster. The opening angle of the outflow is estimated to be ~ 30{\circ}. The
dynamical time-scale of the outflow is estimated to be ~ 100 years. Our results
provide strong support that HW3d harbors an internal massive young star, and
the observed outflow could be tracing a very early phase of star formation. We
also have analyzed Very Large Array (VLA) archive data of 1.3 cm continuum
emission obtained in 1995 and 2006 toward Cepheus A. The comparative result of
the HW3d continuum emission suggests the possibility of the existence of
distinct young stellar objects (YSOs) in HW3d and/or strong variability in one
of their radio continuum emission components.
