  In the conventional view of type II migration, a giant planet migrates inward
in the viscous velocity of the accretion disc in the so-call disc-dominate
case. Recent hydrodynamic simulations, however, showed that planets migrate
with velocities much faster than the viscous one in massive discs. Such fast
migration cannot be explained by the conventional picture. Scardoni et al.
(2020) has recently argued this new picture. By carrying out similar
hydrodynamic simulations, they found that the migration velocity slows down
with time and eventually reaches the prediction by the conventional theory.
They interpreted the fast migration as an initial transient one and concluded
that the conventional type II migration is realised after the transient phase.
We show that the migration velocities obtained by Scardoni et al. (2020) are
consistent with the previous simulations even in the transient phase that they
proposed. We also find that the transient fast migration proposed by Scardoni
et al. (2020) is well described by a new model of Kanagawa et al. (2018). The
new model can appropriately describe significant inward migration during the
initial transient phase that Scardoni et al. (2020) termed. Hence, we conclude
that the time-variation of the transient migration velocity is due to the
changes of the orbital radius of the planet and its background surface density
during the migration.
