  The formation and evolution of the circumstellar disk in the collapsing
molecular cloud is investigated from the prestellar stage resolving both the
molecular cloud core and the protostar itself. In the collapsing cloud, the
first adiabatic core appears prior to the protostar formation. Reflecting the
thermodynamics of the collapsing gas, the first core is much more massive than
the protostar. When the molecular cloud has no angular momentum, the first core
falls onto the protostar and disappears a few years after the protostar
formation. On the other hand, when the molecular cloud has an angular momentum,
the first core does not disappear even after the protostar formation, and
directly evolves into the circumstellar disk with a Keplerian rotation. There
are two paths for the formation of the circumstellar disk. When the initial
cloud has a considerably small rotational energy, two nested disks appear just
after the protostar formation. During the early main accretion phase, the inner
disk increases its size and merges with the outer disk (i.e. first core) to
form a single circumstellar disk with a Keplerian rotation. On the other hand,
when the molecular cloud has a rotational energy comparable to observations, a
single Keplerian disk that corresponds to the first core already exists prior
to the protostar formation. In such a cloud, the first core density gradually
increases, maintaining the Keplerian rotation and forms the protostar inside
it. Thus, the protostar is born in the Keplerian disk. In other words, a
massive disk already exists before the protostar formation. In each case, the
protostar at its formation is already surrounded by a massive circumstellar
disk. The circumstellar disk is about 10-100 times more massive than the
protostar in the main accretion disk. Such disks are favourable sites for the
formation of binary companions and gas-giant planets.
